I recently added dark mode support with a new website for myself, using (prefers-color-scheme: dark) by default, but also allowing the user to switch between light and dark mode if they have JavaScript enabled. It also supports switching image sources in dark mode, so that I can use a dark mode screenshot in dark mode, and light in light. (I haven't yet published the draft article I have using this feature.)
SteamDB also recently added this too, but it also doesn't require JavaScript, as it's based in the cookie. The toggle does use JavaScript if you do have it enabled
I wrote about it at https://chrismorgan.info/blog/dark-theme-implementation/.