Add detection for theme preferences given by the browser.
This works by replacing the "dark mode" setting with a new selector that defaults to "auto" (losing old settings, sorry).
You can try this out by opening the dev console and using the dedicated button (Inspector > sun/moon -shaped in Firefox, Elements > paint brush -shaped in Chromium).
I tried to match the project style but I'm not too good at frontend, I hope this is OK!