Hi It Services,
Indeed the case with RadGridView is a different one because this control uses its own GridViewScrollViewer which internally uses the standard ScrollViewer. You may see this relation within GridViewScrollViewer's template.
As the rest of our themes you may apply Telerik theme to several Microsoft controls such as TextBox, Button, CheckBox, RepeatButton etc. If you want to style the standard ScrollViewer just apply the theme to this control.
For GridViewScrollViewer this technique would not work. May you please share with us a bit more info about your exact scenario? Thus we would be able to provide you with an appropriate solution.
All the best,
the Telerik team
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