With Qt 6 (.?) QMessageBox on macOS is by default shown as a native cocoa message box. If user code has specified a stylesheet to apply to a QMessageBox, the stylesheet will not be applied because the OS window manager is rendering the message box - and has no concept of a stylesheet for a message box.

If a user has specified a stylesheet, that stylesheet should show, thus, the dialog should be rendered as a non-native dialog in that case.

Another (IMHO less optimal) option is that a warning message should be printed telling the user that the stylesheet is not shown due to native message box, and QMessageBox documentation should indicate that stylesheets require the DontUseNativeDialog option.