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_dto_test.dart

@@ -56,6 +56,11 @@ void main() {
       // TODO
     });
 
+    // SystemConfigThemeDto theme
+    test('to test the property `theme`', () async {
+      // TODO
+    });
+
     // SystemConfigThumbnailDto thumbnail
     test('to test the property `thumbnail`', () async {
       // TODO