I have been trying to set the background color of a QTabWidget to black (or any other color for), but have been unsuccessful in doing so.
It seems that you need the option autoFillBackground set and then also set "background-color: black;" in the stylesheet. This then displays it properly in the Designer, but fails in the application.
This answer suggests to enclose it in another QWidget and then use the transparency, but that is a hack around the issue.
How do I set the background color of a QTabWidget via stylesheets?
EDIT
Setting QTabBar { background-color: black; }
results in the following image.