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_theme_dto_test.dart

@@ -0,0 +1,27 @@
+//
+// AUTO-GENERATED FILE, DO NOT MODIFY!
+//
+// @dart=2.12
+
+// ignore_for_file: unused_element, unused_import
+// ignore_for_file: always_put_required_named_parameters_first
+// ignore_for_file: constant_identifier_names
+// ignore_for_file: lines_longer_than_80_chars
+
+import 'package:openapi/api.dart';
+import 'package:test/test.dart';
+
+// tests for ServerThemeDto
+void main() {
+  // final instance = ServerThemeDto();
+
+  group('test ServerThemeDto', () {
+    // SystemConfigThemeDto theme
+    test('to test the property `theme`', () async {
+      // TODO
+    });
+
+
+  });
+
+}