Support color customization from the admin (#2338)

* Add user-customizable theming. Closes #1915

* Prettified Code!

* Add user-customizable theming. Closes #1915

* Add explicit color for page content background

* Prettified Code!

Co-authored-by: gabek <gabek@users.noreply.github.com>

web/interfaces/client-config.model.ts

@@ -12,6 +12,7 @@ export interface ClientConfig {
   chatDisabled: boolean;
   externalActions: any[];
   customStyles: string;
+  appearanceVariables: Map<string, string>;
   maxSocketPayloadSize: number;
   federation: Federation;
   notifications: Notifications;
@@ -58,6 +59,7 @@ export function makeEmptyClientConfig(): ClientConfig {
     chatDisabled: false,
     externalActions: [],
     customStyles: '',
+    appearanceVariables: new Map(),
     maxSocketPayloadSize: 0,
     federation: {
       enabled: false,