[Oxitendwe]

I absolutely agree that this would be a great idea, but it would be much better to solve this at a higher level - the website should provide a light and dark stylesheet, and then the browser should pick based on whether the user has a dark theme or not. Giving a website information about your physical surroundings (GPS coordinates, ambient light, audio input, etc) should be an extremely conscious choice, made only when there is not a better option (Google Maps cannot recalculate your route upon making a wrong turn if it doesn't know where you are, for example).

[Already__Taken]

The server still knows you're requesting those resources. And the browser will always have to request those to check if it can offer you a different layout anyway.

What if I don't want to give you binary light/dark choice. Perhaps the site is flux offering you an app the change your colour balance. It could show you what the app would do now with your existing light levels.