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
