The Focus border of RadComboBox is part of the ButtonChrome control, a control which you will find in the control template of RadComboBox. If you generate the style for the ButtonChrome, you will be able to find and modify the Brushes responsible for any visual state of the control.
You can see the brushes in the resources tab in Blend, as shown in the attached screenshot.
i hope this will be helpful.
the Telerik team
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