fix(web): fix Theme Custom CSS endpoint requiring the user to be logged in as the server admin (#4633)

* fix custom css requiring the user to be the admin and logged in

* move theme api to custom endpoint

* add e2e test

mobile/openapi/test/server_info_api_test.dart

@@ -47,6 +47,11 @@ void main() {
       // TODO
     });
 
+    //Future<ServerThemeDto> getTheme() async
+    test('test getTheme', () async {
+      // TODO
+    });
+
     //Future<ServerPingResponse> pingServer() async
     test('test pingServer', () async {
       // TODO