The screenshot you posted depicts a look that is easily doable with s simple RadComboBox. Is there any specific reason not to use a RadComboBox in your scenario?
If you are using a RadDropDownButton together with an ItemsControl as DropDownContent, you will need a way to close the button when an item has been selected. Furthermore, to customize the RadDropDownButton , you will need to modify the RadDropDownButtonStyle for Metro theme and change the brushes which are applied to each of its visual states. All this, just to resemble a RadComboBox.
Could you share more details on your scenario? Also, please share the XAML mark-up that you use and that you wish to modify.
the Telerik team
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