Just want to say that regardless of implementation method I very much agree @bramus here, that the browser itself is best placed to control colour schemes on websites.
https://www.bram.us/2022/05/25/dark-mode-toggles-should-be-a-browser-feature/
Every time I come to work on such a feature I feel as though I'm hacking on top of the OS and browser. I much prefer the idea of websites stating what they are and are not compatible with and then the browser can decide what's best to offer
! A model where the browser create an easily available UI based on a sites capabilities would be my end-goal as well. https://kilianvalkhof.com/2022/css-html/on-better-browsers-arbitrary-media-queries-and-browser-uis/