(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/server/lib/sendAppPage.ts

@@ -55,6 +55,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     survey: Boolean(process.env.DOC_ID_NEW_USER_INFO),
     tagManagerId: process.env.GOOGLE_TAG_MANAGER_ID,
     activation: getActivation(req as RequestWithLogin | undefined),
+    enableCustomCss: process.env.APP_STATIC_INCLUDE_CUSTOM_CSS === 'true',
     ...extra,
   };
 }