Written
You can add a Class attribute to the Submit button but this is not applied to the Reset and Back buttons if they are enabled (in CFv4 RC3.5). You can add classes with a couple of lines of Javascript:
Drag a Load JS action from the Utilities action group into the On Load event and add this code:
window.addEvent('domready', function() {
$$('[name=reset]')[0].addClass('art-button');
$$('[name=back]')[0].addClass('art-button');
});
Change the highlighted class name to the one you want to apply, this example is for an Artisteer template.