Dark mode flag standardisation for websites

[Romain]

Well, I know this is not the best place (because it concerns ALL browsers), but Firefox is mine and I use this place as a proxy (because I do not know which consortium/place is dedicated to shared standards).

It would be great if the browser companies could define a flag available to the websites (in user-agent header or a specific header, or any other solutions), so that the website owner can provide the appropriate color them.

Since the flag between OS -> browser exists (at least android and Linux/Gnome), it would bring the powerness OS -> (browser ->) website. And when one decides to change at the OS level (night / day or dark/lighty environment, ...), all (compatible) website would 'automatically' adapt.

It might require a wider thinking, because if it should allow dynamic change without reloading a page (to get the new css applied) after switching from one to the other, both css, should be initially provided.

As I think while I am writing, maybe an extension of the css standards would also be a solution, easier/smoother to deploy, detectable by the existing workaround extensions (to disable for those websites).

element {
  color: #123456
  background-color: #456789
  dark-color: #654321
  dark-background-color: #987654
  /* alternatively:
  color-dark: #654321
  background-color-dark: #987654
  */
 }

At the browser niveau:
For some web pages, with dark (e.g. light) mode globally set, one might want to use the light (e.g. dark) theme, so it should be possible to overwrite at the tab level (right click on tab with a light/dark theme switcher button in the context menu or a menu line).