Do you have firebug in firefox? If you do, inspect the element to find out the different class names of the like button. Then when you have the class name you want to style go to your css stylesheet and do as you would normally but at the end of your styles make sure to add an !important. For example:
Are you guys sure he can do that? I don’t think it’s possible to edit content within an iframe since that would be editing another website’s CSS. You can do it with firebug, but I’m pretty sure HTML doesn’t allow that.