Accomodate Website for Dark Mode

[johnzhou721]

What is the problem or limitation you are having?

Dark Mode is becoming increasingly popular. The Furo theme, and therefore the Docs pages, already adjust for it:

However, the main site does not.

Describe the solution you'd like

Adapt the main website pages to Dark Mode.

Describe alternatives you've considered

Status quo and leave a huge contrast when OS is in dark mode.

Additional context

Even though some people may prefer working during the day only, dark mode still works better when eyes are tired in my opinion. (I've actually done this a few years bee-fore... switching phone to dark mode in the light of day...), so the main site should adapt to it.

[johnzhou721]

Also potential problem for beeware-theme, but don't move issue yet as whether the header fits in the the of the docs pages is up for debate.