Would be perfectly reasonable to use js over this feature.

Yes for sure, but my personal policy is not using JS. Anyway my site doesn't have so many visitors ;-) People come, stay for a few seconds and then leave forever.

Hahah old school hate for js. Very good.

However, it would probably be possible with very minimal JS a la how Tailwind does it, toggling a .dark class in the body element and setting the dark mode variables and colors there.

But adding onclick and window.open() JS functions is somehow justifiable, over using simple <a> links? Of course, there might be some specific case you are solving here, so it would be very educational to hear this out.

You can! Trick is to use the checkbox:selected hack to apply styles, in addition to the colour-scheme media query.

It is possible; just too convoluted/subpar imo. You can use a checkbox's check/uncheck status. One such codepen from quick google: https://codepen.io/shadowfaxrodeo/pen/JjbzNMz

Cool thank you for sharing this! I might implement this on my site.