@bix said on Slack:

FWIW, if you’re using Marfa … right now this is giving me a decent-enough quick-and-dirty “dark mode” theme in Chrome and Firefox and Safari. 

@media (prefers-color-scheme: dark) {
  body { filter: invert(100%); }
  img { filter: invert(100%); }
  body { background-color: #000!important; }
}