Add dark mode

Create the ability to toggle on and off dark mode from any page. Add a setting on the accounts page for logged in users to set a preference. Use this preference automatically when a user signs in. Dark mode could potentially become the default depending on the overall implementation.

Start with the accounts page and sign-in flow, then trace how the shared page layout is rendered. The implementation is done when users can toggle dark mode from any page, save the preference from their account, and have it applied automatically after signing in; the default behavior remains to be decided.

Written by the indexing model from the issue text.