feat: add auto dark mode

[oleobal]

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!

[tasn]

Thanks for the contribution! I made some suggestions.

[tasn]

Deployed, thanks!