alt Hacker Newsprefers-color-scheme has shipped for something like 7 years now and not even all of the major sites I frequent which already have a dark theme toggle support it yet. Obviously accessibility is more important than a dark theme (though it can be part of an accessibility story), but if it takes a minute to ship prefers-color-scheme on those existing themed sites and it's been so many years you get the idea of how little it's about when something is easy or not.
The problem with prefers-color-scheme is that it doesn't indicate an actual user preference since the default is not "no preference" (which used to exist as a possible value in at least FF before it was intentionally removed) but rather whatever the OS/browser picked. So any website that wants to choose a "default" appearance but still provide choice to users who actually care cannot use this mechanism at all.