You may change the modal background of RadWindow by using the following CSS:
background: red !important;
If you need to apply a different background color to the "panels" of your project, that resembles a color we have used in the skins of RadWindow, you may use any colorpicker to get the desired color and then apply it with CSS background-color to your control.
If none of these solutions is not your case, I will be waiting for a demo project that explains the result you need to achieve.
the Telerik team