I did not seem to encounter your problems creating a control template for the RadRichTextBox instance and for the Scrollbar. The easiest way to do it in Blend is to select the object from the Objects and Timeline tab, right click it and select Edit template -> Edit a copy. Then you can view the code in the XAML or simply use the designer and alter the properties as you like.
I am attaching a project created entirely in Blend which illustrates how you can set the BackgroundColor of the ScrollBars to red.
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