The background property of RadWindow controls the background of the content of RadWindow. If I get it correct, you refer to the BorderBackground, since you are concerned with the visibility of controls behind RadWindow. To interactively see the effect of changing these properties you can take a look at our online Appearance
example. To programmatically set a background to a transparent color use the following code-snippet:
I hope this will resolve your issue. If this doesn't resolve your case, please get back to us as soon as possible, we will be eager to provide you with additional samples or suggestions.
the Telerik team