Description

prefers-color-scheme is a CSS feature which allows CSS to conditionally set styles based on the user's preference for light or dark mode.

MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-color-scheme
Spec: https://drafts.csswg.org/mediaqueries-5/#prefers-color-scheme

Implementation plan

• Enable prefers-color-scheme stylo#93 enables this feature in Stylo (which is doing all the hard work for us).
• Make Servo compatible with Stylo with prefers-color-scheme support #34423 updates Servo to use that version of Stylo (but hardcodes "light mode").

The remaining work is to:

• Add an embedder event for setting the preferred color scheme
• Plumb this through to the layout thread where it should be set on the device (possibly via a "reflow", similar to window size?)
• Connect this to Winit ThemeChanged events in servoshell. We should also obtain an initial value from winit when creating the window.