Hello,
The above QAT Background is always set to Transparent. It's just that some of our themes for Windows via WindowChrome may have different title bar area backgrounds than the Window's content area.
Usage of RibbonWindow with Ribbon is recommended so that you can get the above QAT to appear in the Window title bar. Otherwise, the QAT will render similar to some of our "inline" samples like the "App Button Content" QuickStart where it is just over the content area.
If you wanted to have a Background on the above QAT, you could try and make an implicit Style targeting the QuickAccessToolBar type and put a Background Setter in the Style.