You may reset the style for the ScrollBars through adding an implicit targeted at the ScrollBar as shown below:
I suggest that you use the Telerik StyleManager, however it does not support this scenario.
You may globally reset the styles for both scrollbars only with NoXAML binaries and pure implicit styles.
I'm attaching you sample project, which demonstrates how to achieve this result.
If you want to read more about this you may check our online documentation for further reference following this link: http://www.telerik.com/help/wpf/styling-apperance-implicit-styles-overview.html
the Telerik team