[PM-5539] Migrate ThemingService (#8219)

* Update ThemingService

* Finish ThemingService

* Lint

* More Tests & Docs

* Refactor to ThemeStateService

* Rename File

* Fix Import

* Remove `type` added to imports

* Update InitServices

* Fix Test

* Remove Unreferenced Code

* Remove Unneeded Null Check

* Add Ticket Link

* Add Back THEMING_DISK

* Fix Desktop

* Create SYSTEM_THEME_OBSERVABLE

* Fix Browser Injection

* Update Desktop Manual Access

* Fix Default Theme

* Update Test

libs/common/src/platform/theming/theme-state.service.ts

@@ -0,0 +1,38 @@
+import { Observable, map } from "rxjs";
+
+import { ThemeType } from "../enums";
+import { GlobalStateProvider, KeyDefinition, THEMING_DISK } from "../state";
+
+export abstract class ThemeStateService {
+  /**
+   * The users selected theme.
+   */
+  selectedTheme$: Observable<ThemeType>;
+
+  /**
+   * A method for updating the current users configured theme.
+   * @param theme The chosen user theme.
+   */
+  setSelectedTheme: (theme: ThemeType) => Promise<void>;
+}
+
+const THEME_SELECTION = new KeyDefinition<ThemeType>(THEMING_DISK, "selection", {
+  deserializer: (s) => s,
+});
+
+export class DefaultThemeStateService implements ThemeStateService {
+  private readonly selectedThemeState = this.globalStateProvider.get(THEME_SELECTION);
+
+  selectedTheme$ = this.selectedThemeState.state$.pipe(map((theme) => theme ?? this.defaultTheme));
+
+  constructor(
+    private globalStateProvider: GlobalStateProvider,
+    private defaultTheme: ThemeType = ThemeType.System,
+  ) {}
+
+  async setSelectedTheme(theme: ThemeType): Promise<void> {
+    await this.selectedThemeState.update(() => theme, {
+      shouldUpdate: (currentTheme) => currentTheme !== theme,
+    });
+  }
+}