Indeed, modifying the control template of the RadButtons
within the RadDataForm
would be the way to go here. Generally, the RadButton has different states( for example - the "Disabled" one) so if you need to apply the same background no matter what the state of the button is, you can remove the different VisualStates from the control template. This might lead to not very user-friendly appearance, though. I have attached a sample for your reference. You can use it as a guidance and modify it to achieve the exact behavior you aim at.
I hope it helps.
Telerik by Progress