@bramus@jensimmons in addition to the privacy concerns, this seems like a booby trap for visitors. If your idea gets implemented, I now have at least 4 places where I need to check for color-scheme manipulation:
OS setting
Browser site-specific settings, with an afterthought interface no doubt
Devtools color-scheme overrides (see bottom of https://chromagen.io for links, they are all confusing and complicated except @FirefoxDevTools)