[Windows][MacOS?] Change title bar color when switching light/dark theme at runtime

[manfromarce]

Description

It would be nice if the window title bar on desktop platforms could change color at runtime, accordingly to the UserAppTheme property which may not be the same as the system theme. Currently title bars only follow the app theme set before launch, e.g. in App.xaml.

Public API Changes

None.

Intended Use-Case

Being able to override the system theme and change between light/dark/system theme at runtime.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.