Although this post is not SPECIFICALLY how to change your specific "option" tag (actually you are not changing anything in the template -- you will be editing the CSS file), it will show you how you can figure this out on your own in about 30 seconds...
For your specific case, change step #4 to:
4. click on a product detail that has a variant
then proceed...
http://forum.x-cart.com/viewtopic.php?p=148575#148575