According to the provided information, it is not very clear what you are trying to achieve: vertical scrollbar in RadGridView or a vertical scrollbar in a panel that hosts several grids.
Note that if the data rows in RadGridView exceed the visible space a vertical scrollbar is shown automatically and it gets the theme from the control.
If you want to use a panel with scrollbar I can suggest to take a look at the RadScrollablePanel. RadScrollablePanel is a variation of RadPanel but customized to support themable scrollbars. The control is composed of two RadScrollBars and a RadPanel that represents a container for other controls and provides the scrolling functionality. The control can be used exactly as a standard panel - any controls can be easily added to it with the standard Visual Studio drag-and-drop routine. It supports all Telerik themes.
I prepared a project example. The result is demonstrated in the attached gif file.
However, if you need further assistance it would be greatly appreciated if you can provide more information about what is the final goal that you would like to achieve.
I hope this information helps. Let me know if you have further questions.
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