refactor(server, web): standardize theme api response (#4664)

* web: standardize theme api response

* revert makefile change that i didn't mean to commit

mobile/openapi/test/server_theme_dto_test.dart

@@ -16,8 +16,8 @@ void main() {
   // final instance = ServerThemeDto();
 
   group('test ServerThemeDto', () {
-    // SystemConfigThemeDto theme
-    test('to test the property `theme`', () async {
+    // String customCss
+    test('to test the property `customCss`', () async {
       // TODO
     });