diff --git a/src/web-ui/src/infrastructure/theme/integrations/MonacoThemeSync.ts b/src/web-ui/src/infrastructure/theme/integrations/MonacoThemeSync.ts
index b2f1f28d..215ad08e 100644
--- a/src/web-ui/src/infrastructure/theme/integrations/MonacoThemeSync.ts
+++ b/src/web-ui/src/infrastructure/theme/integrations/MonacoThemeSync.ts
@@ -10,6 +10,28 @@ const log = createLogger('MonacoThemeSync');
 
 const SEMANTIC_HIGHLIGHTING_RULES = BitFunDarkTheme.rules;
 
+function getBitfunLightMonacoTheme(): monaco.editor.IStandaloneThemeData {
+  return {
+    base: 'vs',
+    inherit: true,
+    rules: SEMANTIC_HIGHLIGHTING_RULES,
+    colors: convertColorsToHex({
+      'focusBorder': '#00000000',
+      'contrastBorder': '#00000000',
+      'diffEditor.insertedTextBorder': '#00000000',
+      'diffEditor.removedTextBorder': '#00000000',
+
+      'editor.selectionBackground': 'rgba(15, 23, 42, 0.14)',
+      'editor.selectionForeground': '#1e293b',
+      'editor.inactiveSelectionBackground': 'rgba(15, 23, 42, 0.09)',
+      'editor.selectionHighlightBackground': 'rgba(15, 23, 42, 0.10)',
+      'editor.selectionHighlightBorder': 'rgba(15, 23, 42, 0.22)',
+      'editor.wordHighlightBackground': 'rgba(15, 23, 42, 0.07)',
+      'editor.wordHighlightStrongBackground': 'rgba(15, 23, 42, 0.11)',
+    }),
+  };
+}
+
  
 function convertToHexColor(color: string): string {
   if (!color) return color;
@@ -85,28 +107,8 @@ export class MonacoThemeSync {
         if (theme.type === 'dark') {
           targetThemeId = 'bitfun-dark';
         } else {
-          
           targetThemeId = 'bitfun-light';
-          
-          monaco.editor.defineTheme('bitfun-light', {
-            base: 'vs',
-            inherit: true,
-            rules: SEMANTIC_HIGHLIGHTING_RULES,
-            colors: convertColorsToHex({
-              'focusBorder': '#00000000',
-              'contrastBorder': '#00000000',
-              'diffEditor.insertedTextBorder': '#00000000',
-              'diffEditor.removedTextBorder': '#00000000',
-              
-              'editor.selectionBackground': 'rgba(15, 23, 42, 0.14)',
-              'editor.selectionForeground': '#1e293b',
-              'editor.inactiveSelectionBackground': 'rgba(15, 23, 42, 0.09)',
-              'editor.selectionHighlightBackground': 'rgba(15, 23, 42, 0.10)',
-              'editor.selectionHighlightBorder': 'rgba(15, 23, 42, 0.22)',
-              'editor.wordHighlightBackground': 'rgba(15, 23, 42, 0.07)',
-              'editor.wordHighlightStrongBackground': 'rgba(15, 23, 42, 0.11)',
-            })
-          });
+          monaco.editor.defineTheme('bitfun-light', getBitfunLightMonacoTheme());
         }
         log.debug('Using builtin theme', { themeId: targetThemeId });
       }
@@ -145,6 +147,38 @@ export class MonacoThemeSync {
   getCurrentThemeId(): string | null {
     return this.currentThemeId;
   }
+
+  /**
+   * Resolves which Monaco theme id should be active for the given app theme
+   * (same rules as {@link syncTheme}).
+   */
+  getTargetMonacoThemeId(theme: ThemeConfig): string {
+    if (theme.monaco) {
+      return theme.id;
+    }
+    return theme.type === 'dark' ? 'bitfun-dark' : 'bitfun-light';
+  }
+
+  /**
+   * Registers BitFun built-in and optional custom Monaco themes on the given Monaco instance.
+   * Use from the Monaco React wrapper `beforeMount` hook so themes exist on the loader's Monaco
+   * before the editor is created (avoids falling back to the default light theme).
+   */
+  registerThemesForEditorInstance(monacoInstance: typeof monaco, theme: ThemeConfig): string {
+    try {
+      monacoInstance.editor.defineTheme('bitfun-dark', BitFunDarkTheme);
+      monacoInstance.editor.defineTheme('bitfun-light', getBitfunLightMonacoTheme());
+
+      if (theme.monaco) {
+        monacoInstance.editor.defineTheme(theme.id, this.convertToMonacoTheme(theme));
+        return theme.id;
+      }
+      return this.getTargetMonacoThemeId(theme);
+    } catch (error) {
+      log.error('registerThemesForEditorInstance failed', error);
+      return 'bitfun-dark';
+    }
+  }
   
    
   private convertToMonacoTheme(theme: ThemeConfig): monaco.editor.IStandaloneThemeData {
diff --git a/src/web-ui/src/tools/mermaid-editor/components/MermaidSyntaxHighlighter.tsx b/src/web-ui/src/tools/mermaid-editor/components/MermaidSyntaxHighlighter.tsx
index 17633d39..d8c8860e 100644
--- a/src/web-ui/src/tools/mermaid-editor/components/MermaidSyntaxHighlighter.tsx
+++ b/src/web-ui/src/tools/mermaid-editor/components/MermaidSyntaxHighlighter.tsx
@@ -6,6 +6,7 @@ import React, { useCallback, useEffect, useMemo, useRef, useState } from 'react'
 import Editor from '@monaco-editor/react';
 import type * as Monaco from 'monaco-editor';
 import { monacoInitManager } from '@/tools/editor/services/MonacoInitManager';
+import { useTheme, themeService, monacoThemeSync } from '@/infrastructure/theme';
 
 export interface MermaidSyntaxHighlighterProps {
   value: string;
@@ -26,6 +27,13 @@ export const MermaidSyntaxHighlighter: React.FC<MermaidSyntaxHighlighterProps> =
   showLineNumbers = true,
   onCursorPositionChange
 }) => {
+  const { theme: appTheme } = useTheme();
+
+  const monacoThemeId = useMemo(() => {
+    const t = appTheme ?? themeService.getCurrentTheme();
+    return t ? monacoThemeSync.getTargetMonacoThemeId(t) : 'bitfun-dark';
+  }, [appTheme]);
+
   const [isReady, setIsReady] = useState(monacoInitManager.isInitialized());
   const [initError, setInitError] = useState<string | null>(null);
   const editorRef = useRef<Monaco.editor.IStandaloneCodeEditor | null>(null);
@@ -60,6 +68,23 @@ export const MermaidSyntaxHighlighter: React.FC<MermaidSyntaxHighlighterProps> =
     };
   }, [isReady]);
 
+  const handleBeforeMount = useCallback((monaco: typeof Monaco) => {
+    const t = appTheme ?? themeService.getCurrentTheme();
+    if (t) {
+      monacoThemeSync.registerThemesForEditorInstance(monaco, t);
+    }
+  }, [appTheme]);
+
+  useEffect(() => {
+    const t = appTheme ?? themeService.getCurrentTheme();
+    const m = monacoInitManager.getMonaco();
+    if (!t || !m) {
+      return;
+    }
+    monacoThemeSync.registerThemesForEditorInstance(m, t);
+    m.editor.setTheme(monacoThemeSync.getTargetMonacoThemeId(t));
+  }, [appTheme]);
+
   const options = useMemo<Monaco.editor.IStandaloneEditorConstructionOptions>(() => ({
     readOnly,
     lineNumbers: showLineNumbers ? 'on' : 'off',
@@ -127,9 +152,10 @@ export const MermaidSyntaxHighlighter: React.FC<MermaidSyntaxHighlighterProps> =
       <Editor
         path={modelPathRef.current}
         language="mermaid"
-        theme="bitfun-dark"
+        theme={monacoThemeId}
         value={value}
         onChange={handleChange}
+        beforeMount={handleBeforeMount}
         onMount={handleMount}
         options={options}
         loading=""