You can customize the appearance of the active or selected RadPane by modifying the DocumentTabChromeStyle. This style is applied to the ButtonChrome controls in RadPane control template.
Attached is a simple sample. Please, search for NOTE to check the two modified spots. In case you wish to emphasize the active pane on PaneGroup level, you can edit the PaneGroupBottomTemplate and its PaneHeader (or the Border containing the PaneHeader, as in the sample).
I hope this will give you a clue on the customizations.
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>