feat: Add 'Restore Default Themes' command for vaults missing theme structure

- New restore_default_themes() Rust fn: seeds both _themes/ and theme/ dirs
- Per-file idempotent: never overwrites existing files with content
- Fixed ensure_vault_themes() to include minimal.md (was missing)
- New 'Restore Default Themes' command in Cmd+K Appearance group
- 3 new Rust tests + 4 new frontend tests
- 1676 frontend tests passed

src-tauri/src/lib.rs

@@ -500,6 +500,12 @@ fn ensure_vault_themes(vault_path: String) -> Result<(), String> {
     theme::ensure_vault_themes(&vault_path)
 }
 
+#[tauri::command]
+fn restore_default_themes(vault_path: String) -> Result<String, String> {
+    let vault_path = expand_tilde(&vault_path);
+    theme::restore_default_themes(&vault_path)
+}
+
 fn log_startup_result(label: &str, result: Result<usize, String>) {
     match result {
         Ok(n) if n > 0 => log::info!("{}: {} files", label, n),
@@ -699,7 +705,8 @@ pub fn run() {
             set_active_theme,
             create_theme,
             create_vault_theme,
-            ensure_vault_themes
+            ensure_vault_themes,
+            restore_default_themes
         ])
         .build(tauri::generate_context!())
         .expect("error while building tauri application")

src-tauri/src/theme.rs

@@ -180,6 +180,7 @@ pub fn ensure_vault_themes(vault_path: &str) -> Result<(), String> {
     let defaults: &[(&str, &str)] = &[
         ("default.md", DEFAULT_VAULT_THEME),
         ("dark.md", DARK_VAULT_THEME),
+        ("minimal.md", MINIMAL_VAULT_THEME),
     ];
     for (name, content) in defaults {
         let path = theme_dir.join(name);
@@ -191,6 +192,34 @@ pub fn ensure_vault_themes(vault_path: &str) -> Result<(), String> {
     Ok(())
 }
 
+/// Restore default themes for a vault: seeds both `_themes/` (JSON) and
+/// `theme/` (markdown notes). Per-file idempotent — never overwrites files
+/// that already have content. Returns an error on read-only filesystems.
+pub fn restore_default_themes(vault_path: &str) -> Result<String, String> {
+    // Seed _themes/ JSON files (per-file idempotent)
+    let themes_dir = Path::new(vault_path).join("_themes");
+    fs::create_dir_all(&themes_dir)
+        .map_err(|e| format!("Failed to create _themes directory: {e}"))?;
+    let json_defaults: &[(&str, &str)] = &[
+        ("default.json", DEFAULT_THEME),
+        ("dark.json", DARK_THEME),
+        ("minimal.json", MINIMAL_THEME),
+    ];
+    for (name, content) in json_defaults {
+        let path = themes_dir.join(name);
+        let needs_write = !path.exists() || fs::metadata(&path).map_or(true, |m| m.len() == 0);
+        if needs_write {
+            fs::write(&path, content)
+                .map_err(|e| format!("Failed to write _themes/{name}: {e}"))?;
+        }
+    }
+
+    // Seed theme/ markdown notes (reuses ensure_vault_themes for consistency)
+    ensure_vault_themes(vault_path)?;
+
+    Ok("Default themes restored".to_string())
+}
+
 /// Create a new vault theme note in `theme/` directory.
 /// Returns the absolute path to the newly created theme note.
 pub fn create_vault_theme(vault_path: &str, name: Option<&str>) -> Result<String, String> {
@@ -949,6 +978,7 @@ mod tests {
         assert!(vault.join("theme").is_dir());
         assert!(vault.join("theme").join("default.md").exists());
         assert!(vault.join("theme").join("dark.md").exists());
+        assert!(vault.join("theme").join("minimal.md").exists());
     }
 
     #[test]
@@ -979,4 +1009,67 @@ mod tests {
         let content = fs::read_to_string(theme_dir.join("default.md")).unwrap();
         assert!(content.contains("#123456"));
     }
+
+    #[test]
+    fn test_restore_default_themes_creates_both_dirs() {
+        let dir = TempDir::new().unwrap();
+        let vault = dir.path().join("vault");
+        fs::create_dir_all(&vault).unwrap();
+        let vp = vault.to_str().unwrap();
+
+        let msg = restore_default_themes(vp).unwrap();
+        assert_eq!(msg, "Default themes restored");
+        // _themes/ JSON files
+        assert!(vault.join("_themes").join("default.json").exists());
+        assert!(vault.join("_themes").join("dark.json").exists());
+        assert!(vault.join("_themes").join("minimal.json").exists());
+        // theme/ markdown notes
+        assert!(vault.join("theme").join("default.md").exists());
+        assert!(vault.join("theme").join("dark.md").exists());
+        assert!(vault.join("theme").join("minimal.md").exists());
+    }
+
+    #[test]
+    fn test_restore_default_themes_is_idempotent() {
+        let dir = TempDir::new().unwrap();
+        let vault = dir.path().join("vault");
+        fs::create_dir_all(&vault).unwrap();
+        let vp = vault.to_str().unwrap();
+
+        restore_default_themes(vp).unwrap();
+        // Modify a theme file to verify it isn't overwritten
+        let custom = "---\nIs A: Theme\nbackground: \"#CUSTOM\"\n---\n";
+        fs::write(vault.join("theme").join("default.md"), custom).unwrap();
+
+        restore_default_themes(vp).unwrap();
+        let content = fs::read_to_string(vault.join("theme").join("default.md")).unwrap();
+        assert!(
+            content.contains("#CUSTOM"),
+            "must not overwrite existing content"
+        );
+    }
+
+    #[test]
+    fn test_restore_default_themes_fills_partial_state() {
+        let dir = TempDir::new().unwrap();
+        let vault = dir.path().join("vault");
+        // Create _themes/ with only default.json, theme/ with only default.md
+        let themes_dir = vault.join("_themes");
+        let theme_dir = vault.join("theme");
+        fs::create_dir_all(&themes_dir).unwrap();
+        fs::create_dir_all(&theme_dir).unwrap();
+        fs::write(themes_dir.join("default.json"), DEFAULT_THEME).unwrap();
+        fs::write(theme_dir.join("default.md"), DEFAULT_VAULT_THEME).unwrap();
+        let vp = vault.to_str().unwrap();
+
+        restore_default_themes(vp).unwrap();
+        // Missing files should now exist
+        assert!(themes_dir.join("dark.json").exists());
+        assert!(themes_dir.join("minimal.json").exists());
+        assert!(theme_dir.join("dark.md").exists());
+        assert!(theme_dir.join("minimal.md").exists());
+        // Existing files should be unchanged
+        let content = fs::read_to_string(theme_dir.join("default.md")).unwrap();
+        assert!(content.contains("Light theme with warm"));
+    }
 }

src/App.tsx

@@ -345,6 +345,19 @@ function App() {
     // 'available' → UpdateBanner handles it automatically
   }, [updateActions, updateStatus.state, setToastMessage])
 
+  const handleRestoreDefaultThemes = useCallback(async () => {
+    if (!resolvedPath) return
+    try {
+      const tauriInvoke = isTauri() ? invoke : mockInvoke
+      const msg = await tauriInvoke<string>('restore_default_themes', { vaultPath: resolvedPath })
+      await vault.reloadVault()
+      await themeManager.reloadThemes()
+      setToastMessage(msg)
+    } catch (err) {
+      setToastMessage(`Failed to restore themes: ${err}`)
+    }
+  }, [resolvedPath, vault, themeManager, setToastMessage])
+
   const commands = useAppCommands({
     activeTabPath: notes.activeTabPath, activeTabPathRef: notes.activeTabPathRef,
     handleCloseTabRef: notes.handleCloseTabRef, tabs: notes.tabs,
@@ -396,6 +409,7 @@ function App() {
     onCheckForUpdates: handleCheckForUpdates,
     onRemoveActiveVault: () => vaultSwitcher.removeVault(vaultSwitcher.vaultPath),
     onRestoreGettingStarted: vaultSwitcher.restoreGettingStarted,
+    onRestoreDefaultThemes: handleRestoreDefaultThemes,
     isGettingStartedHidden: vaultSwitcher.isGettingStartedHidden,
     vaultCount: vaultSwitcher.allVaults.length,
     mcpStatus,

src/hooks/useAppCommands.ts

@@ -62,6 +62,7 @@ interface AppCommandsConfig {
   onCheckForUpdates?: () => void
   onRemoveActiveVault?: () => void
   onRestoreGettingStarted?: () => void
+  onRestoreDefaultThemes?: () => void
   isGettingStartedHidden?: boolean
   vaultCount?: number
   mcpStatus?: string
@@ -172,6 +173,7 @@ export function useAppCommands(config: AppCommandsConfig): CommandAction[] {
     onCheckForUpdates: config.onCheckForUpdates,
     onRemoveActiveVault: config.onRemoveActiveVault,
     onRestoreGettingStarted: config.onRestoreGettingStarted,
+    onRestoreDefaultThemes: config.onRestoreDefaultThemes,
     isGettingStartedHidden: config.isGettingStartedHidden,
     vaultCount: config.vaultCount,
     mcpStatus: config.mcpStatus,

src/hooks/useCommandRegistry.test.ts

@@ -703,6 +703,41 @@ describe('useCommandRegistry', () => {
       expect(cmd!.keywords).toContain('demo')
     })
   })
+
+  describe('restore-default-themes command', () => {
+    it('has restore-default-themes command in Appearance group', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd).toBeDefined()
+      expect(cmd!.label).toBe('Restore Default Themes')
+      expect(cmd!.group).toBe('Appearance')
+      expect(cmd!.enabled).toBe(true)
+    })
+
+    it('is disabled when onRestoreDefaultThemes is not provided', () => {
+      const { result } = renderHook(() => useCommandRegistry(makeConfig()))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd).toBeDefined()
+      expect(cmd!.enabled).toBe(false)
+    })
+
+    it('calls onRestoreDefaultThemes when executed', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      result.current.find(c => c.id === 'restore-default-themes')!.execute()
+      expect(onRestoreDefaultThemes).toHaveBeenCalled()
+    })
+
+    it('has relevant keywords for discoverability', () => {
+      const onRestoreDefaultThemes = vi.fn()
+      const { result } = renderHook(() => useCommandRegistry(makeConfig({ onRestoreDefaultThemes })))
+      const cmd = result.current.find(c => c.id === 'restore-default-themes')
+      expect(cmd!.keywords).toContain('theme')
+      expect(cmd!.keywords).toContain('restore')
+      expect(cmd!.keywords).toContain('default')
+    })
+  })
 })
 
 describe('pluralizeType', () => {

src/hooks/useCommandRegistry.ts

@@ -60,6 +60,7 @@ interface CommandRegistryConfig {
   onOpenTheme?: (themeId: string) => void
   onRemoveActiveVault?: () => void
   onRestoreGettingStarted?: () => void
+  onRestoreDefaultThemes?: () => void
   isGettingStartedHidden?: boolean
   vaultCount?: number
 }
@@ -194,7 +195,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
     themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme,
     onCheckForUpdates,
     onCreateType,
-    onRemoveActiveVault, onRestoreGettingStarted, isGettingStartedHidden, vaultCount,
+    onRemoveActiveVault, onRestoreGettingStarted, onRestoreDefaultThemes, isGettingStartedHidden, vaultCount,
     mcpStatus, onInstallMcp,
   } = config
 
@@ -248,6 +249,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
 
       // Appearance
       ...buildThemeCommands(themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme),
+      { id: 'restore-default-themes', label: 'Restore Default Themes', group: 'Appearance', keywords: ['theme', 'reset', 'restore', 'default', 'fix', 'missing'], enabled: !!onRestoreDefaultThemes, execute: () => onRestoreDefaultThemes?.() },
 
       // Settings
       { id: 'open-settings', label: 'Open Settings', group: 'Settings', shortcut: '⌘,', keywords: ['preferences', 'config'], enabled: true, execute: onOpenSettings },
@@ -271,7 +273,7 @@ export function useCommandRegistry(config: CommandRegistryConfig): CommandAction
     onZoomIn, onZoomOut, onZoomReset, zoomLevel,
     onSelect, onOpenDailyNote, onCloseTab,
     onGoBack, onGoForward, canGoBack, canGoForward,
-    vaultTypes, themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme,
+    vaultTypes, themes, activeThemeId, onSwitchTheme, onCreateTheme, onOpenTheme, onRestoreDefaultThemes,
     onRemoveActiveVault, onRestoreGettingStarted, isGettingStartedHidden, vaultCount,
     mcpStatus, onInstallMcp,
   ])

src/mock-tauri/mock-handlers.ts

@@ -313,6 +313,7 @@ line-height-base: 1.6
     return path
   },
   ensure_vault_themes: (): null => null,
+  restore_default_themes: (): string => 'Default themes restored',
 }
 
 export function addMockEntry(_entry: VaultEntry, content: string): void {