I think the easiest way to style a button in a column in RadGrid is to use a TemplateColumn. You can then add your button, apply whatever CSS styles you need, and then handle the button's events via the Grid's ItemCommandEvent (interrpeting the CommandName and Args for specific processing). I do not there there is a way to directly edit the styles of the HTML button via properties of the GridButtonColumn. It's a good idea, though. Maybe we can enhance this column to in the future expose a property for skinning the button- something like ButtonCssClass.
Let me know if that will work for you.