If the themes for both RadTreeView
have the same names, the scroll bar theme should have been applied as well (if it is loaded of course). If the themes have same names and are not applied, you may have found a bug in RadTreeView.
In order to fix that bug we will need your elaboration - a sample project that demonstrates the issue would be most helpful.
You can explicitly set the theme name for the scroll bars in RadTreeView like the sample below:
|this.radTreeView1.TreeViewElement.ScrollBarThemeName = "MyScrollBarTheme";
Hope this helps. Don't hesitate to contact me if you have further questions.
the Telerik team