In version 2016.2.613.45 of the Office2013 Theme, there is a problem in the control template for the RadTabControl. The ContentBorder element properly gets it's BorderThickness from the TemplatedParent, but the PlacementStates VisualStates override it with hard-coded Thickness (0,1,1,1 in the case of the "Left" Visual State) in their storyboards rather than doing an appropriate transform of the Thickness. This makes it impossible to override the BorderThickness for the control in any state other than the default layout without copying and replacing the entire control template. This appears to be a change in behavior since the (Februray 2016?) release, but I cannot be sure as the old theme files were uninstalled during the upgrade and the telerik.windows.controls.navigation.baml file is not properly decompiled in JustDecompile.