I have seen this in other WPF control, but last time I play with RadTreeView, then when the scrollbar appears.
It use the System Default Theme (in my System it was windows XP Style), it's feel weird because all other aspek of RadTreeView rendered using applied Theme.
I checked this and yes, indeed the ScrollBar does not change its theme as expected. I have not noticed this before under the Vista aero theme but changing it does reveal the bug.
Thank you for bringing our attention to this issue, we should be more careful with these kind of things.
Your telerik points have been updated for your feedback.