Background color of promoted QWidget does not show

I have Qt 4.7 under VC2008 (Windows7-32b) and QtCreator 2.1beta under Mac OSX 10.6.
I placed 2 QWidgets into a tabbed-widget in a MainWindow and changed their background color in Designer (e.g. "background: khaki;"). When I run my program at this point I can see the background colors.
I then promoted the 2 Widgets to my own classes derived from QWidget and the background colors still show in Designer but when I run the program I cannot see the background colors anymore. This is consistent under Mac OSX and Windows.

I tried to set the background in the widget classes with "setStyleSheet()" and in an external .qss stylesheet. But nothing worked. I can though set other CSS styles like "Margin" and "Padding" etc..

"autoFillBackground" has no noticeable effect on the background color of the Widget.

I have no idea if that worked with previous Qt version or not, but since I used 4.7 beta it doesn't.

Added a test case: VC2008 Project.