Throwing this at the bottom of static/style.css fulfilled my needs - it uses the sourcehut color background/nav colors. Not sure if this is what you had in mind.

If this is sufficient, I'd be happy to throw up a patch - anyone else is free to commit this too.

@media (prefers-color-scheme: dark) {
  body {
    background: #212529;
    color: #f8f9fa;
  }

  body a {
    color: #809fff;
  }
	
  button {
    color: #f8f9fa;
  }

  nav {
    background: #131618;
  }
}

Simon Ser referenced this ticket in commit d9c1ec8.

Tweaked a few things and pushed, thanks!