QTabWidget tab palettes don't inherit colors correctly in Qt6

This appears to be a regression in Qt 6.x.

To reproduce the fault, compile and run the attached toy example program under Qt6.x.  Note that when the program runs, the background color of the QTabWidget's tab-headers is dark gray, rather than green.

To observe the correct behavior, compile and run the attached toy example program under Qt5.x.  Note that with Qt5.x, the background color of the QTabWidget's tab-headers is green, as expected.

See also the attached screenshot file showing the fault and the correct behavior.