Theming

[Ralith]

I'm sure this is already on a list somewhere, but for visibility:

Theming allows applications to define a style once and apply it to large numbers of widgets without having to modify pre-existing widget definitions, or even instantiations. Theme information is hence a good candidate for thread-local state or similar, allowing it to be fetched during widget construction without laborious propagation.

Prior art:

• CSS (hopefully this level of complexity is avoidable)
• Qt's QPalette, in particular the suite of named QBrushes it exposes
• Whatever reasoning motivated Gtk to abandon useful theming support
• Do Flutter and/or SwiftUI do anything interesting here?