Thank you for writing.
My advice is to try to do this with modification in the themes, because with one modification in the theme of the Text Box you will change the style of all text boxes in your application. But if there are controls that can not be styling through the modification in the theme due lack of state you should styling them with events. So the best approach in this case is to make mix form both ways of customization.
I hope this helps. Should you have any other questions, I will be glad to assist you
the Telerik team