The tab you wish to change is actually the bottom pane header. Once you generate the style for RadPane, the PaneBottomTemplate will also be generated. This is the template that corresponds to the circled item from your screenshot.
Additionally, if you wish to change the bottom auto-hide area that slides out when you mouse over that same tab, you can do that by generating the style for RadDocking. Drill down that style to find the bottom auto-hide area style and generate and modify it to your needs.
Attached is a basic sample. Please, search for the word NOTE to check the modified spots.
I hope this helps.
the Telerik team
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