How to Customize QMdiSubWindow Title Bar Style in Fusion Theme?

[IPlayGenji6]

The bottom half of my title bar has a shadow effect, but I want to change it to a gradient effect. The first image shows the my style now, and I’d like to achieve the style in the second image.

to

Here is my code:
main.cpp:

QApplication a(argc, argv);    
a.setStyle("Fusion");

Style sheet:

QMdiSubWindow {
selection-background-color: #81c3d6;
}