Dark theme for web UI and Android app

We’ve had requests for a dark theme of the web UI (activitywatch#253) and Android app (aw-android#17). Those two issues are practically the same issue, as the Android app uses the web UI.

While this is admittedly not a priority at this time, we want to see how big the interest is so I’m moving the discussion here to let people vote and discuss.

In the meantime, a decent workaround for desktop users is to use a browser extension like Dark Reader.