1. Set it programmatically. Just put the following code snippet in the Form_Load event handler:
    

   // Set the RadButton color gradient style to solid

   (radButton1.RootElement.Children[0].Children[0] as FillPrimitive).GradientStyle = GradientStyles.Solid;

   // Set the solid color to a color of your choice (in this case trasparent)

   (radButton1.RootElement.Children[0].Children[0] as FillPrimitive).BackColor = Color.Yellow;

   // In case you want to hide the border, choose between these two lines:

   (radButton1.RootElement.Children[0].Children[2] as BorderPrimitive).Visibility = ElementVisibility.Hidden;

   (radButton1.RootElement.Children[0].Children[2] as BorderPrimitive).ForeColor = Color.Transparent;



2. Use Visual Style Builder to make a theme. In the Control Structure panel navigate to RootRadElement >> RadButtonElement >> FillPrimitive. Then change the GradientStyle to Solid and set a color for each of the RadButton states.
   For more info on using Visual Style Builder, please take a look at these video tutorials:
   Visual Style Builder I
   Visual Style Builder II

If you have further questions, do not hesitate to contact us.
 

All the best,

the Telerik team