You should have two styles for the selected item and for the normal items. On the other hand we found an simpler approach on achieving the desired behavior and we create a sample project for you to take a look at. In the project all you need to do is set an ItemTemplate and bind to the IsSelected property of the RadComboBoxItem. It is worth mentioning that this is not the best practice on achieving the desired behavior but it is the simplest one we can provide.
If you have any followup questions feel free to write to us again.
All the best,
the Telerik team