If you are talking about the button created as part of the <input type="file"> element, there are limited options for styling. I do not believe that you will be able to affect the style of the button.
I have read about techniques where the file control is hidden and proxied by stylable elements, but I've never been able to get is working adequately in a cross-browser fashion. (Didn't try too hard though.) You might do a search and see if someone else has been successful.