Existing coloured windows become see-through after upgrading

Existing desktop windows with custom background colours can become see-through after upgrading, even when their window Opacity is 1.0. Previously, these colours blended over the window's opaque theme background, so applications behind the window were not visible.

Existing projects should keep their previous window appearance without having to reselect their colours. Developers must still be able to deliberately create translucent backgrounds, fade entire windows, and use masks for shaped windows.