UserPreferences: Handle in-app dark mode setting being different to OS-level setting

[phoglenix]

Problem:
Web apps often have their own setting/toggle for enabling a dark theme. When they do this, it's independent of the OS-level dark mode setting, and usually set via JS once the web content loads.
This can cause the same problem, in reverse, as this API seeks to solve: the web app loads with one theme/background colour, then switches to another upon load.

I'm not sure what a good solution to this issue would be. Perhaps web apps need a way to inform the OS that they are in dark/light/default mode, so that they will be created with their correct colours instead of using the OS dark/light setting if desired.

[phoglenix]

I think the probable solution to this is a non-declarative (ie. JS) API that lets the app inform the OS that the user has set it to override the OS-level settings (eg. overrideUserPreference("color-scheme" "dark") ). This should not affect the shape of the declarative manifest entries though, so ok to defer until requested by developers.