Hello Ramin,
You could edit the template for the RadWindow. I would suggest you to refer to the following links - http://www.telerik.com/help/silverlight/radwindow-styles-and-templates-template-structure.html and http://www.telerik.com/help/silverlight/radwindow-styles-and-templates-styling-the-radwindow.html
I hope this helps.
Regards,
George
the Telerik team
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