Dark mode can be implement on client side.

Firefox extension: https://addons.mozilla.org/en-US/firefox/addon/dark-backgrou...

Chrome extension: https://chrome.google.com/webstore/detail/dark-mode/dmghijel...

Something to supplant the Ambient Light Sensor API would be great. A boolean value there really goes a long way toward minimizing privacy concerns.

Don't expect any website to support dark theme requests in HTTP headers if most don't even bother to respect Do Not Track headers.

Wouldn’t that add to the fingerprinting surface they just killed.