Revert "fix #12528 (#12536)" (#12548)

This reverts commit a5f0b5ec.

packages/frontend/src/scripts/theme.ts

@@ -44,7 +44,7 @@ export const getBuiltinThemes = () => Promise.all(
 		'd-cherry',
 		'd-ice',
 		'd-u0',
-	].map(name => import(`${__dirname}/../themes/${name}.json5`).then(({ default: _default }): Theme => _default)),
+	].map(name => import(`../themes/${name}.json5`).then(({ default: _default }): Theme => _default)),
 );
 
 export const getBuiltinThemesRef = () => {