diff --git a/cmd/rawmark/main.go b/cmd/rawmark/main.go
index 6b22534..f92de21 100644
--- a/cmd/rawmark/main.go
+++ b/cmd/rawmark/main.go
@@ -95,6 +95,7 @@ func run(addr string) error {
 	go cleanupSessions(ctx, st)
 
 	srv := api.New(v, am, st, hub, idx)
+	srv.ThemesDir = filepath.Join(dataDir, "themes")
 	srv.OnSetup = func() { os.Remove(codeFile) }
 	go srv.FollowMoves(ctx)
 	httpServer := &http.Server{
diff --git a/internal/api/api.go b/internal/api/api.go
index fe2de22..3f697b9 100644
--- a/internal/api/api.go
+++ b/internal/api/api.go
@@ -39,6 +39,8 @@ type Server struct {
 	settingsMu sync.Mutex
 	// OnSetup 在首次初始化完成后调用（用于删除启动码文件）
 	OnSetup func()
+	// ThemesDir 指向数据目录中的主题目录；为空时只有内置主题。
+	ThemesDir string
 }
 
 func New(v *vault.Vault, a *auth.Manager, st *store.Store, hub *events.Hub, index *search.Index) *Server {
@@ -89,6 +91,8 @@ func (s *Server) routes() []route {
 
 		{"GET /api/v1/settings", authed, s.getSettings},
 		{"PATCH /api/v1/settings", sessionOnly, s.patchSettings},
+		{"GET /api/v1/themes", authed, s.listThemes},
+		{"GET /api/v1/themes/{id}", authed, s.getTheme},
 		{"GET /api/v1/shares", sessionOnly, s.listShares},
 		{"POST /api/v1/shares", sessionOnly, s.createShare},
 		{"DELETE /api/v1/shares/{token}", sessionOnly, s.deleteShare},
@@ -97,6 +101,7 @@ func (s *Server) routes() []route {
 		{"DELETE /api/v1/tokens/{id}", sessionOnly, s.deleteToken},
 
 		{"GET /api/v1/public/shares/{token}", public, s.publicShare},
+		{"GET /api/v1/public/shares/{token}/appearance", public, s.publicAppearance},
 		{"POST /api/v1/public/shares/{token}/unlock", public, s.unlockShare},
 		{"GET /api/v1/public/shares/{token}/assets/{path...}", public, s.publicShareAsset},
 	}
diff --git a/internal/api/api_test.go b/internal/api/api_test.go
index f09170d..bf9f6ea 100644
--- a/internal/api/api_test.go
+++ b/internal/api/api_test.go
@@ -55,7 +55,9 @@ func newFixture(t *testing.T) *fixture {
 		t.Fatal(err)
 	}
 	t.Cleanup(func() { idx.Close() })
-	srv := httptest.NewServer(New(v, am, st, events.NewHub(), idx).Handler(fstest.MapFS{}))
+	server := New(v, am, st, events.NewHub(), idx)
+	server.ThemesDir = filepath.Join(dir, "themes")
+	srv := httptest.NewServer(server.Handler(fstest.MapFS{}))
 	t.Cleanup(srv.Close)
 	f := &fixture{t: t, url: srv.URL, root: v.Root(), store: st, admin: newClient()}
 	if res := f.do(f.admin, "POST", "/api/v1/auth/login", `{"password":"password123"}`, nil); res.StatusCode != 200 {
diff --git a/internal/api/openapi.json b/internal/api/openapi.json
index fa1a72b..6d7f3d6 100644
--- a/internal/api/openapi.json
+++ b/internal/api/openapi.json
@@ -1510,6 +1510,164 @@
           }
         }
       }
+    },
+    "/themes": {
+      "get": {
+        "summary": "列出主题（含内置默认主题）",
+        "tags": [
+          "外观"
+        ],
+        "responses": {
+          "200": {
+            "description": "主题列表",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "themes": {
+                      "type": "array",
+                      "items": {
+                        "$ref": "#/components/schemas/Theme"
+                      }
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    },
+    "/themes/{id}": {
+      "get": {
+        "summary": "读取主题 CSS",
+        "tags": [
+          "外观"
+        ],
+        "parameters": [
+          {
+            "name": "id",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "主题信息与 CSS",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "allOf": [
+                    {
+                      "$ref": "#/components/schemas/Theme"
+                    },
+                    {
+                      "type": "object",
+                      "properties": {
+                        "css": {
+                          "type": "string"
+                        }
+                      }
+                    }
+                  ]
+                }
+              }
+            }
+          },
+          "404": {
+            "description": "主题不存在或文件不完整"
+          }
+        }
+      }
+    },
+    "/public/shares/{token}/appearance": {
+      "get": {
+        "summary": "读取有效分享使用的外观配置",
+        "description": "仅返回明暗模式、字号、正文宽度、主题 CSS 和启用的 CSS 片段；密码分享需先解锁，不返回其他设置，不加载插件。",
+        "tags": [
+          "分享"
+        ],
+        "security": [
+          {}
+        ],
+        "parameters": [
+          {
+            "name": "token",
+            "in": "path",
+            "required": true,
+            "schema": {
+              "type": "string"
+            }
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "公开的外观配置",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "type": "object",
+                  "properties": {
+                    "colorMode": {
+                      "type": "string",
+                      "enum": [
+                        "system",
+                        "light",
+                        "dark"
+                      ]
+                    },
+                    "fontSize": {
+                      "type": "integer"
+                    },
+                    "contentWidth": {
+                      "type": "integer"
+                    },
+                    "themeCSS": {
+                      "type": "string"
+                    },
+                    "cssSnippets": {
+                      "type": [
+                        "array",
+                        "null"
+                      ],
+                      "items": {
+                        "type": "object",
+                        "properties": {
+                          "id": {
+                            "type": "string"
+                          },
+                          "name": {
+                            "type": "string"
+                          },
+                          "css": {
+                            "type": "string"
+                          },
+                          "enabled": {
+                            "type": "boolean"
+                          }
+                        }
+                      }
+                    },
+                    "theme": {
+                      "type": "string"
+                    }
+                  }
+                }
+              }
+            }
+          },
+          "401": {
+            "description": "需要访问密码"
+          },
+          "404": {
+            "description": "分享不存在、已过期或已撤销"
+          }
+        }
+      }
     }
   },
   "components": {
@@ -1672,6 +1830,25 @@
             "description": "Unix 毫秒，0 为从未使用"
           }
         }
+      },
+      "Theme": {
+        "type": "object",
+        "required": [
+          "id",
+          "name",
+          "version"
+        ],
+        "properties": {
+          "id": {
+            "type": "string"
+          },
+          "name": {
+            "type": "string"
+          },
+          "version": {
+            "type": "string"
+          }
+        }
       }
     }
   }
diff --git a/internal/api/themes.go b/internal/api/themes.go
new file mode 100644
index 0000000..cd36b47
--- /dev/null
+++ b/internal/api/themes.go
@@ -0,0 +1,165 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/http"
+	"os"
+	"regexp"
+)
+
+var themeIDPattern = regexp.MustCompile(`^[a-z0-9][a-z0-9_-]{0,63}$`)
+
+type themeManifest struct {
+	ID      string `json:"id"`
+	Name    string `json:"name"`
+	Version string `json:"version"`
+}
+
+func readThemeFile(root *os.Root, name string, limit int64) ([]byte, error) {
+	f, err := root.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	if !info.Mode().IsRegular() {
+		return nil, fmt.Errorf("主题文件必须为普通文件")
+	}
+	data, err := io.ReadAll(io.LimitReader(f, limit+1))
+	if err != nil {
+		return nil, err
+	}
+	if int64(len(data)) > limit {
+		return nil, fmt.Errorf("主题文件过大")
+	}
+	return data, nil
+}
+
+func (s *Server) readTheme(id string) (themeManifest, string, error) {
+	var manifest themeManifest
+	if !themeIDPattern.MatchString(id) || id == "default" || s.ThemesDir == "" {
+		return manifest, "", fs.ErrNotExist
+	}
+	root, err := os.OpenRoot(s.ThemesDir)
+	if err != nil {
+		return manifest, "", err
+	}
+	defer root.Close()
+	dir, err := root.OpenRoot(id)
+	if err != nil {
+		return manifest, "", err
+	}
+	defer dir.Close()
+	data, err := readThemeFile(dir, "manifest.json", 16<<10)
+	if err != nil {
+		return manifest, "", err
+	}
+	if err := json.Unmarshal(data, &manifest); err != nil {
+		return manifest, "", fmt.Errorf("主题 %s 的 manifest.json 无法解析：%w", id, err)
+	}
+	if manifest.ID != id || manifest.Name == "" || manifest.Version == "" {
+		return manifest, "", fmt.Errorf("主题 %s 的 id、name 或 version 无效", id)
+	}
+	css, err := readThemeFile(dir, "theme.css", 256<<10)
+	return manifest, string(css), err
+}
+
+func (s *Server) listThemes(w http.ResponseWriter, r *http.Request) {
+	themes := []themeManifest{{ID: "default", Name: "默认", Version: "1"}}
+	if s.ThemesDir != "" {
+		entries, err := os.ReadDir(s.ThemesDir)
+		if err != nil && !errors.Is(err, fs.ErrNotExist) {
+			internalError(w, r, err)
+			return
+		}
+		for _, entry := range entries {
+			if !entry.IsDir() || !themeIDPattern.MatchString(entry.Name()) || entry.Name() == "default" {
+				continue
+			}
+			manifest, _, err := s.readTheme(entry.Name())
+			if err != nil {
+				internalError(w, r, err)
+				return
+			}
+			themes = append(themes, manifest)
+		}
+	}
+	writeJSON(w, 200, map[string]any{"themes": themes})
+}
+
+func (s *Server) getTheme(w http.ResponseWriter, r *http.Request) {
+	manifest, css, err := s.readTheme(r.PathValue("id"))
+	if errors.Is(err, fs.ErrNotExist) {
+		writeError(w, 404, "主题不存在或文件不完整")
+		return
+	}
+	if err != nil {
+		internalError(w, r, err)
+		return
+	}
+	writeJSON(w, 200, struct {
+		themeManifest
+		CSS string `json:"css"`
+	}{manifest, css})
+}
+
+// 只有有效分享的访客可读取外观；不返回账户、插件和其他应用设置。
+func (s *Server) publicAppearance(w http.ResponseWriter, r *http.Request) {
+	if _, ok := s.openShare(w, r); !ok {
+		return
+	}
+	settings, err := s.readSettings()
+	if err != nil {
+		internalError(w, r, err)
+		return
+	}
+	out := map[string]any{}
+	for _, key := range []string{"colorMode", "fontSize", "contentWidth"} {
+		if value, ok := settings[key]; ok {
+			out[key] = value
+		}
+	}
+	var snippets []struct {
+		ID      string `json:"id"`
+		Name    string `json:"name"`
+		CSS     string `json:"css"`
+		Enabled bool   `json:"enabled"`
+	}
+	if value, ok := settings["cssSnippets"]; ok {
+		if err := json.Unmarshal(value, &snippets); err != nil {
+			internalError(w, r, err)
+			return
+		}
+	}
+	active := snippets[:0]
+	for _, snippet := range snippets {
+		if snippet.Enabled {
+			active = append(active, snippet)
+		}
+	}
+	out["cssSnippets"] = active
+	var theme string
+	if value, ok := settings["theme"]; ok {
+		if err := json.Unmarshal(value, &theme); err != nil {
+			internalError(w, r, err)
+			return
+		}
+	}
+	if theme != "" && theme != "default" {
+		_, css, err := s.readTheme(theme)
+		if err != nil {
+			internalError(w, r, err)
+			return
+		}
+		out["themeCSS"] = css
+	}
+	out["theme"] = theme
+	writeJSON(w, 200, out)
+}
diff --git a/internal/api/themes_test.go b/internal/api/themes_test.go
new file mode 100644
index 0000000..4851d0c
--- /dev/null
+++ b/internal/api/themes_test.go
@@ -0,0 +1,76 @@
+package api
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestThemeAndShareAppearanceBoundaries(t *testing.T) {
+	f := newFixture(t)
+	dir := filepath.Join(filepath.Dir(f.root), "themes", "paper")
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	for name, data := range map[string]string{
+		"manifest.json": `{"id":"paper","name":"纸张","version":"1"}`,
+		"theme.css":     `:root { --rm-bg: #faf6ed; }`,
+	} {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(data), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	guest := newClient()
+	if code := f.status(guest, "GET", "/api/v1/themes/paper", "", nil); code != 401 {
+		t.Fatalf("匿名用户不应读取主题，得到 %d", code)
+	}
+	themes := decode[struct{ Themes []themeManifest }](t, f.do(f.admin, "GET", "/api/v1/themes", "", nil))
+	if len(themes.Themes) != 2 || themes.Themes[1].ID != "paper" {
+		t.Fatalf("主题列表错误：%+v", themes.Themes)
+	}
+	theme := decode[map[string]string](t, f.do(f.admin, "GET", "/api/v1/themes/paper", "", nil))
+	if !strings.Contains(theme["css"], "#faf6ed") {
+		t.Fatal("未读取到主题 CSS")
+	}
+	f.write("笔记.md", "正文")
+	f.do(f.admin, "PATCH", "/api/v1/settings", `{"theme":"paper","colorMode":"dark","fontSize":18,"privateSetting":"不公开","cssSnippets":[{"id":"a","name":"启用","css":"body { color: red }","enabled":true},{"id":"b","name":"停用","css":"不公开片段","enabled":false}]}`, nil)
+	sh := decode[shareView](t, f.do(f.admin, "POST", "/api/v1/shares", `{"path":"笔记.md","password":"访问密码"}`, nil))
+	base := "/api/v1/public/shares/" + sh.Token
+	if code := f.status(guest, "GET", base+"/appearance", "", nil); code != 401 {
+		t.Fatalf("未解锁不能读取外观，得到 %d", code)
+	}
+	f.do(guest, "POST", base+"/unlock", `{"password":"访问密码"}`, nil)
+	res := f.do(guest, "GET", base+"/appearance", "", nil)
+	raw, err := io.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var appearance map[string]any
+	if err := json.Unmarshal(raw, &appearance); err != nil {
+		t.Fatal(err)
+	}
+	if res.StatusCode != 200 || appearance["colorMode"] != "dark" || !strings.Contains(appearance["themeCSS"].(string), "#faf6ed") {
+		t.Fatalf("分享外观错误：%s", raw)
+	}
+	if strings.Contains(string(raw), "不公开") || len(appearance["cssSnippets"].([]any)) != 1 {
+		t.Fatalf("分享泄露了其他设置或停用片段：%s", raw)
+	}
+	f.do(f.admin, "DELETE", "/api/v1/shares/"+sh.Token, "", nil)
+	if code := f.status(guest, "GET", base+"/appearance", "", nil); code != 404 {
+		t.Fatalf("撤销分享后不能读取外观，得到 %d", code)
+	}
+	// CSS 符号链接不能读取主题目录之外的应用数据。
+	if err := os.Remove(filepath.Join(dir, "theme.css")); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(filepath.Join(filepath.Dir(f.root), "rawmark.db"), filepath.Join(dir, "theme.css")); err != nil {
+		t.Fatal(err)
+	}
+	if code := f.status(f.admin, "GET", "/api/v1/themes/paper", "", nil); code != http.StatusInternalServerError {
+		t.Fatalf("应拒绝越界链接，得到 %d", code)
+	}
+}
diff --git a/web/src/app/AppearanceSettings.vue b/web/src/app/AppearanceSettings.vue
new file mode 100644
index 0000000..2babe22
--- /dev/null
+++ b/web/src/app/AppearanceSettings.vue
@@ -0,0 +1,104 @@
+<script setup lang="ts">
+import { computed, onMounted, ref } from 'vue'
+import { api, ApiError, type Theme } from './api'
+import { appearancePaused, type CssSnippet } from '../themes/appearance'
+import { loadSettings, reloadTheme, settings, updateSettings, type Settings } from './settings'
+import { toast } from './workspace'
+
+const themes = ref<Theme[]>([{ id: 'default', name: '默认', version: '1' }])
+const error = ref('')
+const busy = ref(false)
+const editing = ref<string | null>(null)
+const draft = ref({ name: '', css: '' })
+const missingTheme = computed(() => !themes.value.some((theme) => theme.id === settings.theme))
+
+async function refreshThemes() {
+  error.value = ''
+  try {
+    themes.value = await api.themes()
+    await reloadTheme()
+  }
+  catch (err) { error.value = err instanceof ApiError ? err.message : '读取主题失败，请检查网络' }
+}
+onMounted(refreshThemes)
+
+async function change(patch: Partial<Settings>): Promise<boolean> {
+  busy.value = true
+  try {
+    await updateSettings(patch)
+    return true
+  } catch (err) {
+    toast('保存外观失败：' + (err instanceof ApiError ? err.message : '网络错误，请检查连接'))
+    void loadSettings().catch((err) => toast('重新读取外观失败：' + (err instanceof ApiError ? err.message : '网络错误')))
+    return false
+  } finally { busy.value = false }
+}
+
+function edit(snippet?: CssSnippet) {
+  editing.value = snippet?.id ?? ''
+  draft.value = { name: snippet?.name ?? '', css: snippet?.css ?? '' }
+}
+
+async function saveSnippet() {
+  if (!draft.value.name.trim()) return
+  const snippets = settings.cssSnippets.map((s) => ({ ...s }))
+  const existing = snippets.find((s) => s.id === editing.value)
+  if (existing) Object.assign(existing, draft.value, { name: draft.value.name.trim() })
+  else {
+    // getRandomValues 在局域网 HTTP 开发页面也可用，randomUUID 需要安全上下文。
+    const id = [...crypto.getRandomValues(new Uint32Array(4))].map((n) => n.toString(16).padStart(8, '0')).join('')
+    snippets.push({ ...draft.value, name: draft.value.name.trim(), id, enabled: true })
+  }
+  if (await change({ cssSnippets: snippets })) editing.value = null
+}
+
+async function toggle(snippet: CssSnippet) {
+  await change({ cssSnippets: settings.cssSnippets.map((s) => s.id === snippet.id ? { ...s, enabled: !s.enabled } : { ...s }) })
+}
+
+async function remove(snippet: CssSnippet) {
+  if (!window.confirm(`删除 CSS 片段“${snippet.name}”？`)) return
+  if (await change({ cssSnippets: settings.cssSnippets.filter((s) => s.id !== snippet.id) })) {
+    if (editing.value === snippet.id) editing.value = null
+  }
+}
+
+async function reset() {
+  if (!await change({ colorMode: 'system', theme: 'default', fontSize: 16, contentWidth: 760,
+    cssSnippets: settings.cssSnippets.map((s) => ({ ...s, enabled: false })) })) return
+  toast('已恢复默认外观，CSS 片段已停用并保留')
+}
+</script>
+
+<template>
+  <p v-if="appearancePaused" class="rm-appearance-note">已临时停用主题和 CSS。可恢复默认外观后重新打开页面。</p>
+  <div class="rm-setting">
+    <label for="rm-color-mode" class="rm-setting-label">明暗模式</label>
+    <select id="rm-color-mode" :value="settings.colorMode" :disabled="busy" @change="change({ colorMode: ($event.target as HTMLSelectElement).value as Settings['colorMode'] })">
+      <option value="system">跟随系统</option><option value="light">浅色</option><option value="dark">深色</option>
+    </select>
+  </div>
+  <div class="rm-setting">
+    <label for="rm-theme-select" class="rm-setting-label">主题</label>
+    <select id="rm-theme-select" :value="settings.theme" :disabled="busy" @change="change({ theme: ($event.target as HTMLSelectElement).value })">
+      <option v-for="theme in themes" :key="theme.id" :value="theme.id">{{ theme.name }}</option>
+      <option v-if="missingTheme" :value="settings.theme">{{ settings.theme }}（不可用）</option>
+    </select>
+    <button class="rm-btn" :disabled="busy" @click="refreshThemes">刷新列表</button>
+  </div>
+  <p v-if="error" class="rm-appearance-note rm-auth-error">{{ error }}</p>
+  <p class="rm-appearance-note">主题从服务器数据目录的 themes 文件夹读取；主题和已启用的 CSS 也会应用到分享页。</p>
+  <div class="rm-setting"><span class="rm-setting-label">CSS 片段</span><button class="rm-btn" :disabled="busy" @click="edit()">新建片段</button></div>
+  <div v-for="snippet in settings.cssSnippets" :key="snippet.id" class="rm-snippet-row">
+    <label><input type="checkbox" :checked="snippet.enabled" :disabled="busy" @change="toggle(snippet)" />{{ snippet.name }}</label>
+    <button class="rm-btn" :disabled="busy" @click="edit(snippet)">编辑</button>
+    <button class="rm-btn rm-btn-danger" :disabled="busy" @click="remove(snippet)">删除</button>
+  </div>
+  <form v-if="editing !== null" class="rm-snippet-form" @submit.prevent="saveSnippet">
+    <label>片段名称<input v-model="draft.name" aria-label="片段名称" maxlength="100" required /></label>
+    <label>CSS<textarea v-model="draft.css" aria-label="CSS" rows="9" spellcheck="false" placeholder=":root { --rm-accent: #805ad5; }" /></label>
+    <div><button class="rm-btn" :disabled="busy" type="submit">保存并应用</button> <button class="rm-btn" type="button" @click="editing = null">取消</button></div>
+  </form>
+  <div class="rm-setting"><span class="rm-setting-label">默认外观</span><button class="rm-btn" :disabled="busy" @click="reset">恢复默认外观</button></div>
+  <p class="rm-appearance-note">若自定义样式导致界面无法操作，可在地址末尾加 <code>?reset-appearance</code>，临时停用后再恢复默认。</p>
+</template>
diff --git a/web/src/app/SettingsDialog.vue b/web/src/app/SettingsDialog.vue
index d34dc85..6d6102a 100644
--- a/web/src/app/SettingsDialog.vue
+++ b/web/src/app/SettingsDialog.vue
@@ -3,6 +3,7 @@ import { computed, onBeforeUnmount, onMounted, ref } from 'vue'
 import { api, ApiError } from './api'
 import { allCommands, formatKeys, runCommand } from './commands'
 import Icon, { type IconName } from './Icon.vue'
+import AppearanceSettings from './AppearanceSettings.vue'
 import { CONTENT_WIDTHS, FONT_SIZES, loadSettings, settings, updateSettings, type Settings } from './settings'
 import SharePanel from './SharePanel.vue'
 import TokensPanel from './TokensPanel.vue'
@@ -106,6 +107,7 @@ async function changePassword() {
         </div>
         <div class="rm-settings-content">
           <template v-if="section === 'appearance'">
+            <AppearanceSettings />
             <div class="rm-setting">
               <span class="rm-setting-label">字号</span>
               <select :value="settings.fontSize" aria-label="字号" @change="change({ fontSize: Number(($event.target as HTMLSelectElement).value) })">
diff --git a/web/src/app/api.ts b/web/src/app/api.ts
index 9411366..ba49c72 100644
--- a/web/src/app/api.ts
+++ b/web/src/app/api.ts
@@ -7,6 +7,12 @@ export interface Entry {
   mtime: number
 }
 
+export interface Theme {
+  id: string
+  name: string
+  version: string
+}
+
 export interface TrashItem {
   id: string
   path: string
@@ -97,6 +103,8 @@ export const api = {
   changePassword: (current: string, next: string) => request('POST', '/api/v1/auth/password', { json: { current, next } }),
 
   settings: () => json<Record<string, unknown>>(request('GET', '/api/v1/settings')),
+  themes: async () => (await json<{ themes: Theme[] }>(request('GET', '/api/v1/themes'))).themes,
+  theme: (id: string) => json<Theme & { css: string }>(request('GET', '/api/v1/themes/' + encodeURIComponent(id))),
   // 按顶层键合并，值为 null 的键被删除
   patchSettings: (patch: Record<string, unknown>) => json<Record<string, unknown>>(request('PATCH', '/api/v1/settings', { json: patch })),
 
diff --git a/web/src/app/app.css b/web/src/app/app.css
index cf2d1ed..343d4be 100644
--- a/web/src/app/app.css
+++ b/web/src/app/app.css
@@ -1,25 +1,4 @@
 /* 应用外壳样式，颜色与字体沿用 themes/base.css 中的 --rm-* 变量 */
-:root {
-  --rm-sidebar-bg: #f9f9f9;
-  --rm-sidebar-width: 260px;
-  --rm-hover: rgba(0, 0, 0, 0.045);
-  --rm-active: rgba(66, 184, 131, 0.14);
-  --rm-danger: #e5484d;
-  --rm-warn-bg: #fff8e6;
-  --rm-warn-text: #8a5a00;
-  --rm-shadow: 0 8px 24px rgba(0, 0, 0, 0.12);
-}
-@media (prefers-color-scheme: dark) {
-  :root {
-    --rm-sidebar-bg: #161618;
-    --rm-hover: rgba(255, 255, 255, 0.06);
-    --rm-active: rgba(66, 211, 146, 0.16);
-    --rm-warn-bg: #2f2716;
-    --rm-warn-text: #f0c060;
-    --rm-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
-  }
-}
-
 html,
 body {
   height: 100%;
@@ -534,3 +513,14 @@ button {
   .rm-switcher-item { height: 44px; font-size: 15px; }
   .rm-trash-list li { flex-wrap: wrap; }
 }
+
+/* 主题与 CSS 编辑 */
+.rm-appearance-note { margin: 8px 20px 16px; font-size: 13px; color: var(--rm-text-muted); overflow-wrap: anywhere; }
+.rm-snippet-row { display: flex; align-items: center; gap: 8px; padding: 6px 20px; }
+.rm-snippet-row label { display: flex; align-items: center; gap: 8px; flex: 1; min-width: 0; overflow-wrap: anywhere; font-size: 14px; }
+.rm-snippet-row input { accent-color: var(--rm-accent); flex: none; }
+.rm-snippet-form { display: flex; flex-direction: column; gap: 12px; padding: 12px 20px; }
+.rm-snippet-form label { display: flex; flex-direction: column; gap: 6px; font-size: 13px; }
+.rm-snippet-form input, .rm-snippet-form textarea { box-sizing: border-box; width: 100%; min-width: 0; padding: 8px; border: 1px solid var(--rm-divider); border-radius: 6px; background: var(--rm-bg); color: var(--rm-text); font: inherit; }
+.rm-snippet-form textarea { font-family: var(--rm-font-mono); resize: vertical; }
+@media (pointer: coarse) { .rm-snippet-row .rm-btn, .rm-snippet-form .rm-btn { min-height: 44px; } .rm-snippet-form input, .rm-snippet-form textarea { font-size: 16px; } }
diff --git a/web/src/app/settings.ts b/web/src/app/settings.ts
index 0f93e3a..f137ccb 100644
--- a/web/src/app/settings.ts
+++ b/web/src/app/settings.ts
@@ -2,6 +2,7 @@
 // 本地缓存上次的值，启动时先套用，避免页面先按默认值显示再跳变
 import { reactive } from 'vue'
 import { api } from './api'
+import { applyAppearance, appearancePaused, sanitizeAppearance, type Appearance } from '../themes/appearance'
 
 export const FONT_SIZES = [14, 15, 16, 17, 18, 20, 22]
 // 0 为全宽
@@ -15,7 +16,7 @@ export const CONTENT_WIDTHS = [
 export const SIDEBAR_WIDTH = { min: 200, max: 480 }
 
 const DEFAULTS = { fontSize: 16, contentWidth: 760, sidebarWidth: 260 }
-export type Settings = typeof DEFAULTS
+export type Settings = typeof DEFAULTS & Appearance
 
 const CACHE_KEY = 'rawmark.settings'
 
@@ -26,6 +27,7 @@ function sanitize(raw: Record<string, unknown>): Settings {
   const pick = <T>(v: unknown, allowed: T[], def: T) => (allowed.includes(v as T) ? (v as T) : def)
   return {
     fontSize: pick(raw.fontSize, FONT_SIZES, DEFAULTS.fontSize),
+    ...sanitizeAppearance(raw),
     contentWidth: pick(
       raw.contentWidth,
       CONTENT_WIDTHS.map((w) => w.value),
@@ -38,6 +40,25 @@ function sanitize(raw: Record<string, unknown>): Settings {
   }
 }
 
+let currentTheme = ''
+let currentCSS = ''
+let appearanceRequest = 0
+
+async function applySettingsAppearance() {
+  const request = ++appearanceRequest
+  const theme = settings.theme
+  if (appearancePaused || theme === 'default') {
+    currentTheme = theme
+    currentCSS = ''
+  } else if (theme !== currentTheme) {
+    const data = await api.theme(theme)
+    if (request !== appearanceRequest) return
+    currentCSS = data.css
+    currentTheme = theme
+  }
+  applyAppearance({ ...settings }, currentCSS)
+}
+
 function applied(next: Settings) {
   Object.assign(settings, next)
   localStorage.setItem(CACHE_KEY, JSON.stringify(settings))
@@ -48,13 +69,30 @@ function applied(next: Settings) {
 }
 
 applied({ ...settings })
+// 缓存只提前应用明暗模式；主题和片段等服务器确认后再应用。
+applyAppearance({ ...settings, theme: 'default', cssSnippets: [] }, '')
 
 export async function loadSettings() {
   applied(sanitize(await api.settings()))
+  await applySettingsAppearance()
+}
+
+export async function reloadTheme() {
+  currentTheme = ''
+  await applySettingsAppearance()
 }
 
 /** 修改设置：先在本地生效再保存，保存失败时由调用方提示并重新载入 */
 export async function updateSettings(patch: Partial<Settings>) {
   applied({ ...settings, ...patch })
-  await api.patchSettings(patch)
+  // 设置请求串行提交，防止快速切换和片段编辑的旧请求覆盖新值。
+  await saveSettings(patch)
+  await applySettingsAppearance()
+}
+
+let saveQueue: Promise<unknown> = Promise.resolve()
+function saveSettings(patch: Partial<Settings>) {
+  const pending = saveQueue.then(() => api.patchSettings(patch))
+  saveQueue = pending.catch(() => {})
+  return pending
 }
diff --git a/web/src/app/workspace.ts b/web/src/app/workspace.ts
index 83fea57..d49df55 100644
--- a/web/src/app/workspace.ts
+++ b/web/src/app/workspace.ts
@@ -11,6 +11,7 @@ import { connectEvents, type VaultEvent } from './events'
 import { basename, dirname, joinPath, NOTE_EXT, resolveAssetUrl } from './paths'
 import { loadSettings, settings } from './settings'
 import { trackVisualViewport } from './viewport'
+import { appearancePaused } from '../themes/appearance'
 
 export type TabStatus = 'loading' | 'ready' | 'conflict' | 'deleted' | 'error'
 
@@ -53,7 +54,7 @@ export const ws = reactive({
   // 快速切换/命令面板的初始输入，null 为关闭；以 > 开头时列出命令
   switcher: null as string | null,
   // 打开的设置分组，null 为关闭；index 为从通用入口打开（手机上先显示分组列表，电脑上显示第一组）
-  settings: null as SettingsSection | 'index' | null,
+  settings: (appearancePaused ? 'appearance' : null) as SettingsSection | 'index' | null,
   // 分享当前笔记的对话框（笔记路径）
   sharing: null as string | null,
   // 「移动到…」对话框（笔记或文件夹路径）
@@ -798,6 +799,7 @@ export async function startWorkspace() {
     () => [settings.fontSize, settings.contentWidth, settings.sidebarWidth],
     () => view?.requestMeasure(),
   )
+  window.addEventListener('rawmark-appearance', () => requestAnimationFrame(() => view?.requestMeasure()))
   // 页面隐藏/关闭前尽力发出待保存的修改（部分浏览器关闭标签页时只触发 pagehide）
   document.addEventListener('visibilitychange', () => {
     if (document.visibilityState === 'hidden') void flushAll(true)
diff --git a/web/src/editor/widgets.ts b/web/src/editor/widgets.ts
index 115066f..d8a24b2 100644
--- a/web/src/editor/widgets.ts
+++ b/web/src/editor/widgets.ts
@@ -223,6 +223,7 @@ export class MathWidget extends WidgetType {
 }
 
 const mermaidSeqs = new WeakMap<HTMLElement, number>()
+const mermaidListeners = new WeakMap<HTMLElement, () => void>()
 let mermaidSeq = 0
 
 /** Mermaid 图表，editOffset 含义同 MathWidget */
@@ -243,14 +244,28 @@ export class MermaidWidget extends WidgetType {
     else editOnMousedown(el, view, this.editOffset)
     el.textContent = '图表渲染中…'
     this.render(el, view)
+    const update = () => this.render(el, view)
+    mermaidListeners.set(el, update)
+    window.addEventListener('rawmark-appearance', update)
     return el
   }
   // 只复用预览元素：从渲染结果切到编辑时，编辑器会尝试用新组件更新旧元素（旧元素带点击编辑的监听）
   updateDOM(dom: HTMLElement, view: EditorView) {
     if (this.editOffset !== null || !dom.classList.contains('cm-rm-preview')) return false
     this.render(dom, view)
+    const old = mermaidListeners.get(dom)
+    if (old) window.removeEventListener('rawmark-appearance', old)
+    const update = () => this.render(dom, view)
+    mermaidListeners.set(dom, update)
+    window.addEventListener('rawmark-appearance', update)
     return true
   }
+  destroy(dom: HTMLElement) {
+    const update = mermaidListeners.get(dom)
+    if (update) window.removeEventListener('rawmark-appearance', update)
+    mermaidListeners.delete(dom)
+    mermaidSeqs.delete(dom)
+  }
   // 渲染是异步的，连续输入时只采用最后一次的结果，并保留旧图直到新图就绪，避免闪烁
   private render(el: HTMLElement, view: EditorView) {
     const seq = ++mermaidSeq
diff --git a/web/src/markdown/render.ts b/web/src/markdown/render.ts
index 18a064c..88ba37d 100644
--- a/web/src/markdown/render.ts
+++ b/web/src/markdown/render.ts
@@ -16,19 +16,18 @@ export async function renderMath(el: HTMLElement, tex: string, displayMode: bool
 let mermaidLoad: Promise<typeof import('mermaid').default> | undefined
 let mermaidSeq = 0
 function loadMermaid() {
-  mermaidLoad ??= import('mermaid').then(({ default: mermaid }) => {
-    const dark = matchMedia('(prefers-color-scheme: dark)').matches
-    // strict：图表中的 HTML 会被清理，点击事件等交互被禁用
-    const config: MermaidConfig = { startOnLoad: false, securityLevel: 'strict', theme: dark ? 'dark' : 'default' }
-    mermaid.initialize(config)
-    return mermaid
-  })
+  mermaidLoad ??= import('mermaid').then(({ default: mermaid }) => mermaid)
   return mermaidLoad
 }
 
 /** 渲染 Mermaid 图表，返回 SVG 标记；语法错误时抛出带说明的 Error */
-export async function renderMermaid(code: string): Promise<string> {
+async function renderDiagram(code: string): Promise<string> {
   const mermaid = await loadMermaid()
+  const mode = document.documentElement.dataset.rmColorMode
+  const dark = mode === 'dark' || (mode === undefined && matchMedia('(prefers-color-scheme: dark)').matches)
+  // 配置与渲染串行进行，避免切换模式时并发图表读到另一个模式的全局配置。
+  const config: MermaidConfig = { startOnLoad: false, securityLevel: 'strict', theme: dark ? 'dark' : 'default' }
+  mermaid.initialize(config)
   const id = `rm-mermaid-${++mermaidSeq}`
   try {
     return (await mermaid.render(id, code)).svg
@@ -39,6 +38,13 @@ export async function renderMermaid(code: string): Promise<string> {
   }
 }
 
+let renderQueue: Promise<unknown> = Promise.resolve()
+export function renderMermaid(code: string): Promise<string> {
+  const result = renderQueue.then(() => renderDiagram(code))
+  renderQueue = result.catch(() => {})
+  return result
+}
+
 let emojiLoad: Promise<Map<string, string>> | undefined
 
 /** 按 GitHub/Typora 的名称查找 emoji，如 smile → 😄；未知名称返回 undefined */
diff --git a/web/src/share/main.ts b/web/src/share/main.ts
index c4a2e08..234cb3e 100644
--- a/web/src/share/main.ts
+++ b/web/src/share/main.ts
@@ -2,6 +2,7 @@
 import { requestTokens } from '../highlight/client'
 import { renderMarkdown } from '../markdown/html'
 import { hydrate, renderMermaid } from '../markdown/render'
+import { applyAppearance, sanitizeAppearance } from '../themes/appearance'
 import '../themes/base.css'
 import './share.css'
 
@@ -22,12 +23,28 @@ function message(text: string) {
 async function load() {
   let res: Response
   try {
-    res = await fetch(api, { credentials: 'same-origin' })
+    res = await fetch(api, { credentials: 'same-origin', cache: 'no-store' })
   } catch {
     message('网络错误，请检查连接后刷新')
     return
   }
-  if (res.ok) render((await res.json()) as SharedNote)
+  if (res.ok) {
+    const note = (await res.json()) as SharedNote
+    let appearanceError = false
+    try {
+      const response = await fetch(api + '/appearance', { cache: 'no-store' })
+      if (!response.ok) throw new Error('读取外观失败')
+      const appearance = await response.json() as Record<string, unknown>
+      const rootStyle = document.documentElement.style
+      if (typeof appearance.fontSize === 'number') rootStyle.setProperty('--rm-font-size', appearance.fontSize + 'px')
+      if (typeof appearance.contentWidth === 'number') rootStyle.setProperty('--rm-content-width', appearance.contentWidth ? appearance.contentWidth + 'px' : 'none')
+      applyAppearance(sanitizeAppearance(appearance), typeof appearance.themeCSS === 'string' ? appearance.themeCSS : '')
+    } catch {
+      appearanceError = true
+    }
+    render(note)
+    if (appearanceError) root.prepend(Object.assign(document.createElement('p'), { className: 'rm-share-error', textContent: '外观加载失败，可刷新重试' }))
+  }
   else if (res.status === 401) askPassword()
   else if (res.status === 404) message('分享不存在、已过期或已被撤销')
   else message(`加载失败（${res.status}）`)
@@ -127,6 +144,7 @@ async function decorateCode(code: HTMLElement) {
   if (lang === 'mermaid') {
     const figure = document.createElement('div')
     figure.className = 'rm-share-mermaid'
+    figure.dataset.code = text
     try {
       figure.innerHTML = await renderMermaid(text)
       code.parentElement!.replaceWith(figure)
@@ -157,4 +175,11 @@ async function decorateCode(code: HTMLElement) {
   code.replaceChildren(out)
 }
 
+window.addEventListener('rawmark-appearance', () => {
+  for (const el of root.querySelectorAll<HTMLElement>('.rm-share-mermaid')) {
+    const code = el.dataset.code
+    if (code !== undefined) void renderMermaid(code).then((svg) => { el.innerHTML = svg }).catch(() => { el.textContent = '图表渲染失败，请刷新重试' })
+  }
+})
+
 void load()
diff --git a/web/src/themes/appearance.ts b/web/src/themes/appearance.ts
new file mode 100644
index 0000000..199711c
--- /dev/null
+++ b/web/src/themes/appearance.ts
@@ -0,0 +1,75 @@
+export interface CssSnippet {
+  id: string
+  name: string
+  css: string
+  enabled: boolean
+}
+
+export interface Appearance {
+  colorMode: 'system' | 'light' | 'dark'
+  theme: string
+  cssSnippets: CssSnippet[]
+}
+
+export const appearancePaused = new URLSearchParams(location.search).has('reset-appearance')
+
+export function sanitizeAppearance(raw: Record<string, unknown>): Appearance {
+  return {
+    colorMode: raw.colorMode === 'light' || raw.colorMode === 'dark' ? raw.colorMode : 'system',
+    theme: typeof raw.theme === 'string' && /^[a-z0-9][a-z0-9_-]{0,63}$/.test(raw.theme) ? raw.theme : 'default',
+    cssSnippets: Array.isArray(raw.cssSnippets) ? raw.cssSnippets.filter((s): s is CssSnippet =>
+      s !== null && typeof s === 'object' && typeof s.id === 'string' && typeof s.name === 'string' &&
+      typeof s.css === 'string' && typeof s.enabled === 'boolean',
+    ).map((s) => ({ id: s.id, name: s.name, css: s.css, enabled: s.enabled })) : [],
+  }
+}
+
+let current: Appearance = { colorMode: 'system', theme: 'default', cssSnippets: [] }
+let themeCSS = ''
+const darkMode = matchMedia('(prefers-color-scheme: dark)')
+let rendered = ''
+
+function style(id: string, css: string) {
+  let el = document.getElementById(id) as HTMLStyleElement | null
+  if (!el) {
+    el = document.createElement('style')
+    el.id = id
+    document.head.append(el)
+  }
+  if (el.textContent !== css) el.textContent = css
+}
+
+function render() {
+  const mode = appearancePaused ? 'system' : current.colorMode
+  const dark = mode === 'system' ? darkMode.matches : mode === 'dark'
+  const signature = JSON.stringify([dark, appearancePaused ? '' : [current.theme, themeCSS, current.cssSnippets]])
+  if (signature === rendered) return
+  rendered = signature
+  const root = document.documentElement
+  root.dataset.rmColorMode = dark ? 'dark' : 'light'
+  root.dataset.rmTheme = appearancePaused ? 'default' : current.theme
+  style('rm-theme', appearancePaused ? '' : themeCSS)
+  // 独立样式元素避免某个片段的未闭合语法吞掉其他片段。
+  document.querySelectorAll('style[data-rm-snippet]').forEach((el) => el.remove())
+  if (!appearancePaused) for (const snippet of current.cssSnippets) {
+    if (!snippet.enabled) continue
+    const el = document.createElement('style')
+    el.dataset.rmSnippet = snippet.id
+    el.textContent = snippet.css
+    document.head.append(el)
+  }
+  const bg = getComputedStyle(root).getPropertyValue('--rm-bg').trim()
+  for (const meta of document.querySelectorAll<HTMLMetaElement>('meta[name="theme-color"]')) {
+    meta.removeAttribute('media')
+    meta.content = bg
+  }
+  window.dispatchEvent(new Event('rawmark-appearance'))
+}
+
+darkMode.addEventListener('change', render)
+
+export function applyAppearance(next: Appearance, css: string) {
+  current = next
+  themeCSS = css
+  render()
+}
diff --git a/web/src/themes/base.css b/web/src/themes/base.css
index 9fde8e7..ba9682b 100644
--- a/web/src/themes/base.css
+++ b/web/src/themes/base.css
@@ -3,6 +3,16 @@
  * 代码块与行内代码配色取自 vuejs.org（@vue/theme + Shiki github-dark）。
  */
 :root {
+  color-scheme: light;
+  --rm-sidebar-bg: #f9f9f9;
+  --rm-sidebar-width: 260px;
+  --rm-hover: rgba(0, 0, 0, 0.045);
+  --rm-active: rgba(66, 184, 131, 0.14);
+  --rm-danger: #e5484d;
+  --rm-warn-bg: #fff8e6;
+  --rm-warn-text: #8a5a00;
+  --rm-shadow: 0 8px 24px rgba(0, 0, 0, 0.12);
+
   --rm-font-text: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'PingFang SC', 'Hiragino Sans GB',
     'Microsoft YaHei', 'Noto Sans CJK SC', 'Helvetica Neue', sans-serif;
   --rm-font-mono: ui-monospace, 'SF Mono', 'SFMono-Regular', Menlo, Monaco, 'Cascadia Mono', Consolas,
@@ -27,8 +37,30 @@
   --rm-highlight-bg: rgba(255, 213, 0, 0.35);
 }
 
+:root:where([data-rm-color-mode="dark"]) {
+  color-scheme: dark;
+  --rm-bg: #1a1a1a;
+  --rm-text: rgba(255, 255, 255, 0.87);
+  --rm-text-muted: rgba(235, 235, 235, 0.6);
+  --rm-divider: rgba(84, 84, 84, 0.48);
+  --rm-accent: #42d392;
+  --rm-accent-strong: #42b883;
+  --rm-inline-code-bg: #2f2f2f;
+  --rm-inline-code-text: #aac8e4;
+  --rm-code-bg: #242424;
+  --rm-highlight-bg: rgba(255, 213, 0, 0.28);
+
+  --rm-sidebar-bg: #161618;
+  --rm-hover: rgba(255, 255, 255, 0.06);
+  --rm-active: rgba(66, 211, 146, 0.16);
+  --rm-warn-bg: #2f2716;
+  --rm-warn-text: #f0c060;
+  --rm-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
+}
+
 @media (prefers-color-scheme: dark) {
-  :root {
+  :root:where(:not([data-rm-color-mode])) {
+    color-scheme: dark;
     --rm-bg: #1a1a1a;
     --rm-text: rgba(255, 255, 255, 0.87);
     --rm-text-muted: rgba(235, 235, 235, 0.6);
@@ -39,6 +71,13 @@
     --rm-inline-code-text: #aac8e4;
     --rm-code-bg: #242424;
     --rm-highlight-bg: rgba(255, 213, 0, 0.28);
+
+    --rm-sidebar-bg: #161618;
+    --rm-hover: rgba(255, 255, 255, 0.06);
+    --rm-active: rgba(66, 211, 146, 0.16);
+    --rm-warn-bg: #2f2716;
+    --rm-warn-text: #f0c060;
+    --rm-shadow: 0 8px 24px rgba(0, 0, 0, 0.5);
   }
 }