When you set a style to the RadPaneGroup, it basically overrides the theme, if any.
I suggest you use the resources directly from the Windows7 theme and apply any customizations to that particular RadPaneGroupStyle.
Also, when you take the pane group out of the normal flow of RadDocking, it becomes a ToolWindow. To customize the header of the dragged pane group, you need to customize the ToolWindowStyle. You can check more details here
I hope this helps.
the Telerik team
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