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/infra/entities/system-config.entity.ts

@@ -90,6 +90,8 @@ export enum SystemConfigKey {
 
   TRASH_ENABLED = 'trash.enabled',
   TRASH_DAYS = 'trash.days',
+
+  THEME_CUSTOM_CSS = 'theme.customCss',
 }
 
 export enum TranscodePolicy {
@@ -221,4 +223,7 @@ export interface SystemConfig {
     enabled: boolean;
     days: number;
   };
+  theme: {
+    customCss: string;
+  };
 }