§ According to web usabiliti experts you should never "@!#$ around" with the standard GUI looks. People understand that they should click a button and are prepared to live with the fact that the submitbutton is grey and ugly.
§ It is not a PHP question. It's a CSS issue perhaps. Go to http://www.wasp.overnetdata.net/ -> Signup
(don't confirm your order unless you have a lot of money) ...and look at what I've made to the buttons in IE. (Not my decision, but they like it).
§ If you're already using GET and have a looong and confusion querystring, why be bothered about the submitbutton.
§ You can use image instead of type="submit" in HTML.
Take note:
www.useit.com