Follow
https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme Curiously, there is a CSS media feature for testing whether someone prefers light or dark mode (which is useful for accessibility purposes), although it's a bit of an obscure one.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-CH-Prefers-Color-Scheme There is also this header but it only works in Chrome (and I guess it doesn't matter how someone does it).