I've made custom radio boxes before. Let me see if I can explain this clearly:

I started out with three instances of a radio button - off, active and selected. For the HTML, I made a <div> with a CSS background image of the radio button and a hidden radio button element inside. Using JavaScript, I wrote a quick little function that switched the background and told the radio button to be selected when the user clicked on the <div>.

Understand? The only problem is that is uses JavaScript. It wouldn't be too difficult to make it still work without the extra styling if JS was disabled, though.