fix: isTextmateTheme type guard for multi-theme validation

Extract isTextmateTheme as reusable type guard function and use it
inside .some() callback to properly check each theme value.

Previously the boolean was computed once on themeInput and incorrectly
reused inside .some() where it should check each individual value.

Amp-Thread-ID: https://ampcode.com/threads/T-019b0c55-150d-7167-815e-ed3140b50974
Co-authored-by: Amp <amp@ampcode.com>

Author: AVGVSTVS96

package/src/lib/theme.ts

@@ -23,31 +23,34 @@ export type ThemeShikiOptions =
   | CodeOptionsSingleTheme<BundledTheme>
   | CodeOptionsMultipleThemes<BundledTheme>;
 
+const isTextmateTheme = (value: unknown): value is ThemeRegistrationAny =>
+  typeof value === 'object' &&
+  value !== null &&
+  'tokenColors' in value &&
+  Array.isArray((value as ThemeRegistrationAny).tokenColors);
+
 export function resolveTheme(
   themeInput: Theme | Themes
 ): ThemeResolution {
-  const isTextmateTheme =
-    typeof themeInput === 'object' &&
-    'tokenColors' in themeInput &&
-    Array.isArray(themeInput.tokenColors);
+  const inputIsTextmateTheme = isTextmateTheme(themeInput);
 
   // Non-textmate objects are assumed to be multi-theme configs
   const isMultiThemeConfig =
     typeof themeInput === 'object' &&
     themeInput !== null &&
-    !isTextmateTheme;
+    !inputIsTextmateTheme;
 
   const validMultiThemeObj =
     typeof themeInput === 'object' &&
     themeInput !== null &&
-    !isTextmateTheme &&
+    !inputIsTextmateTheme &&
     Object.entries(themeInput).some(
       ([key, value]) =>
         key &&
         value &&
         key.trim() !== '' &&
         value !== '' &&
-        (typeof value === 'string' || isTextmateTheme)
+        (typeof value === 'string' || isTextmateTheme(value))
     );
 
   if (isMultiThemeConfig) {

package/tests/hook.test.tsx

@@ -562,6 +562,41 @@ describe('useShikiHighlighter Hook', () => {
       // Should use custom prefix
       expect(styledToken?.htmlStyle?.['--theme-dark']).toBeDefined();
     });
+
+    test('multi-theme with two TextMate theme objects works', async () => {
+      const code = 'const x = 1;';
+      // Both values are TextMate theme objects (no strings)
+      const lightTheme = {
+        name: 'test-light',
+        tokenColors: [
+          { scope: 'keyword', settings: { foreground: '#ff0000' } },
+        ],
+      };
+      const darkTheme = {
+        name: 'test-dark',
+        tokenColors: [
+          { scope: 'keyword', settings: { foreground: '#00ff00' } },
+        ],
+      };
+      const themes = { light: lightTheme, dark: darkTheme };
+
+      const { result } = renderHook(() =>
+        useShikiHighlighter(code, 'javascript', themes, {
+          outputFormat: 'tokens',
+        })
+      );
+
+      // Wait for highlighting to complete
+      await waitFor(() => {
+        const tokensResult = result.current as TokensResult;
+        // Should have theme name from our custom theme, not fallback
+        expect(tokensResult.themeName).not.toBe('');
+      });
+
+      const tokensResult = result.current as TokensResult;
+      // Verify it used our themes, not DEFAULT_THEMES fallback
+      expect(tokensResult.tokens.length).toBeGreaterThan(0);
+    });
   });
 
   describe('TokenRenderer DOM Output', () => {