(core) Add dark mode to user preferences

Summary:
Adds initial implementation of dark mode. Preferences for dark mode are
available on the account settings page. Dark mode is currently a beta feature
as there are still some small bugs to squash and a few remaining UI elements
to style.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Subscribers: paulfitz, jarek

Differential Revision: https://phab.getgrist.com/D3587

app/client/ui/AppUI.ts

@@ -105,6 +105,7 @@ function pagePanelsHome(owner: IDisposableOwner, appModel: AppModel, app: App) {
     headerMain: createTopBarHome(appModel),
     contentMain: createDocMenu(pageModel),
     contentTop: buildHomeBanners(appModel),
+    testId,
   });
 }