Control Template
RadMaskedInput's visual appearance is defined through a Control Template. In order to customize the way the MaskedInput looks, you would need to take the default ControlTemplate and modify it.
This topic gives an overview of the ControlTemplate of the MaskedInput control, so you can copy it to your app and make changes.
The original ControlTemplate
The XAML defined below relies on the following namespace to be set:
Finally, use the custom ControlTemplate as a StaticResource on any instance of RadMaskedInput: