By default the RadTabControl ExpressionDark style has such border but its color is gray and it might not be noticeable in some case (see the attached picture - tabcontrol.png). To achieve the effect you are looking for in the ExpressionDark theme you will need to extract the ControlTemplate
of RadTabControl and modify the corresponding Border control. You can see the concrete Border element in the attached tabcontrol-visual-tree.png picture.
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