As far as I understand you need to customize one of the states of the RadPanelBarItem. This can be done in fairly easy way by following the steps described in this article
. You should open the Telerik.Windows.Controls.Navigation.xaml dictionary and you should be looking for the style with x:Key attribute set to "RadPanelbarItemBaseStyle". Furthermore you should customize its Template or its ChildItemsTemplate depending on your needs.
I hope this information will get you started.
Pavel R. Pavlov
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