feat(web): custom stylesheets (#4602)

* add initial ui and api definitions for stylesheets

* proper saving

* make custom css work

* add textarea

* rebuild api

* run prettier

* add typecast

* update typings

* move css accordion to be sorted alphabetically

* set content-type properly

* rename stylesheets to theme

* fix server test

mobile/openapi/test/system_config_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 SystemConfigThemeDto
+void main() {
+  // final instance = SystemConfigThemeDto();
+
+  group('test SystemConfigThemeDto', () {
+    // String customCss
+    test('to test the property `customCss`', () async {
+      // TODO
+    });
+
+
+  });
+
+}