(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/common/gristUrls.ts

@@ -558,6 +558,9 @@ export interface GristLoadConfig {
 
   // String to append to the end of the HTML document.title
   pageTitleSuffix?: string;
+
+  // If custom CSS should be included in the head of each page.
+  enableCustomCss?: boolean;
 }
 
 export const HideableUiElements = StringUnion("helpCenter", "billing", "templates", "multiSite", "multiAccounts");