To customize the appearance of the RadButtons you can use the standard properties that are provided by the Control class, like Background, Foreground, Effect etc.
Also you can use the RadButtons specific properties:
CornerRadius - specifies the value for the radius of the button's corners.
IsBackgroundVisible - specifies whether the background is visible, when the button is in its Normal state.
Applying a style to the button is the most efficient way to customize its appearance. The styles can be reused between buttons, which will make your code easier to maintain.
By using a Style you can also entirely change the appearance of the button. This is done by modifying its ControlTemplate. To learn more about styling the RadButtons read here.