Restore the built-in Shiki GitHub Dark code palette

internal/api/themes/black.css

@@ -16,8 +16,8 @@
   --rm-shadow: 0 12px 36px rgba(0, 0, 0, 0.6);
   --rm-inline-code-bg: #202127;
   --rm-inline-code-text: #f19a58;
-  --rm-code-bg: #263238;
-  --rm-code-text: #abb2bf;
+  --rm-code-bg: #24292e;
+  --rm-code-text: #e1e4e8;
   --rm-highlight-bg: rgba(232, 196, 119, 0.25);
 }