Site dark mode

[rossabaker]

From http4s/http4s#5313:

Support dark mode:
You can configure a second set of colors for the theme and for highlighting so that the site adapts to the user's OS/browser preferences.

I don't personally use dark mode, but honoring user preferences , so I'm 👍 on this.

I think it's done like this.

[armanbilge]

Since http4s has its own theme, this issue doesn't float up to sbt-typelevel.