The MSDN documentation on Button Styles will probably be of help to you. It gives an example of a WPF Button Template that you should be able to edit to your requirements. You can put the Xaml in Windows.Resources if the style is only going to be used on that Form, or you can edit the Application.xaml file and put the Style information in the Application.Resources Section if it it is to be used for the entire application.
Modified above style link to give you an example:
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