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The NonEditable template of the ComboBox is represented by a ButtonChrome control. You have to edit its MouseOver, Pressed, etc states. You can find a short video illustrating that in this forum post: http://www.telerik.com/community/forums/silverlight/menu/radmenu-hover-behaviors.aspx#1312865
It is about a MenuItem, but the approach is the same, as the MenuItem also uses the ButtonChrome control.
Hope this helps. Please let us know if you have additional questions.
the Telerik team
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