GroupBox captions show the wrong background on coloured windows

A desktop GroupBox caption shows an opaque rectangle in the default theme colour when the surrounding window has a custom background colour. For example, on a red window the caption has a white background in the light theme or a dark background in the dark theme.

The caption should blend into the surrounding surface while retaining the gap in the GroupBox's outline around the text. The designer preview and running desktop application should behave consistently, including when the GroupBox is inside another container.