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

server/src/domain/system-config/system-config.core.ts

@@ -114,6 +114,9 @@ export const defaults = Object.freeze<SystemConfig>({
     enabled: true,
     days: 30,
   },
+  theme: {
+    customCss: '',
+  },
 });
 
 export enum FeatureFlag {