The RadRichTextBox can be styled by creating an appropriate Style and setting it to the Style property of the control.
You have two options:
To create an empty style and set it up on your own.
To copy the default style of the control and modify it.
This topic will show you how to perform the second one.
To copy the default style, load your project in Expression Blend and open the User Control that holds the RadRichTextBox. In the 'Objects and Timeline' pane, select the RadRichTextBox you want to style. From the menu choose Object -> Edit Style -> Edit a Copy. You will be prompted for the name of the style and where to be placed.
If you choose to define the style in Application, it would be available for the entire application. This allows you to define a style only once and then reuse it where needed.
After clicking 'OK', Expression Blend will generate the default style of the RadRichTextBox control in the Resources section of your User Control. The properties available for the style will be loaded in the 'Properties' pane and you will be able to modify their default values. You can also edit the generated XAML in the XAML View or in Visual Studio. For example, you can change the background of the RadRichTextBox by modifying the Background property.