Thank you for your report, but the described issue is not a bug. Setting the BackgroundVisibility property to "Hidden" removes the background and it becomes white. On the other hand the foreground of the TabItem's headers in Windows8Theme is also white and that makes them invisible. To make them visible you can set the TabItem's foreground to a color different than the white color.
Unfortunately, we found another issue while looking into your scenario - in Windows8 theme, setting the TabItem's foreground property doesn't work. This bug is logged in our PITS
, where you can check its status and vote for it.
Also I update your telerik points.
the Telerik team