Sorry for late.
First, you can change blue frame by prefferences dialogbox.
- Tree: Window/View – Colors
- Grid: Window colors – PopupFrame
The behavior of this popup window is different from other window like floating Player View. It cannot lock to stay on desktop, it cannot close forced by clicking some button.
So I thought it should be a different color from other window (But blue… it may have been a little adventure(^^;).