VIRTUEMART: Styling the ADD TO CART button

The following will remove the ADD To CART text entirely (This could be used if the button was an image):

components/com_virtuemart/themes/default/templates/product_details/includes/addtocart_form.tpl.php

Other modifications can be made using this line of code.
class: Gives the button a CSS class
value: gives the button text (ie: Add To Cart)