Is it possible the repo has a different dist.css than your demo website (https://devaultcss.avalos.me/)? On the website I see the darkmode taking effect and
dist.css, but not in the repo yet.
Thanks for taking the effort!
Actually, CSS can detect dark-mode itself too: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme
I think I see sr.ht using it too, and as I though you basically copy that importing it afresh should not be too much effort. Or do you do some manual merging/editing?