I am not sure I understand you right. You have TextBox that is multiline and shows vertical scrollbar. And the scrollbar (or ScrollViewer that holds it) does not get the telerik theme as you set it?
Have you tried putting the telerik:StyleManager.Theme
on the TextBlock. It should pass the theme to all internal controls including the ScrollViewer and the scroll viewer will set the theme on the ScrollBars.
the Telerik team
Let us know about your Windows Phone 7 application built with RadControls and we will help you promote it. Learn more>>