fix: resolve test failures for theme builder integration

- Replace hardcoded text-white and text-red-* with CSS variables in theme-editor
- Add theme-builder migration entry to housekeeping matrix
- Update legacy page counts from 137 to 138 in housekeeping tests
- Add /admin/theme-builder to content test prefixes

src/features/housekeeping/migration/content.test.ts

@@ -18,6 +18,7 @@ const CONTENT_PREFIXES = [
 	"/admin/writeable-boxes",
 	"/admin/email-templates",
 	"/admin/theme",
+	"/admin/theme-builder",
 	"/admin/favicon",
 	"/admin/translations",
 ] as const;