....sorry but it is Mayashop's button. But any how what are you trying to change it to? The current one is an image and the hover you see IS the correct CSS property to change. because it reveals on hover...
.shop-ribbon .products li .buttons .add-to-cart, .shop-traditional .products.ribbon li .buttons .add-to-cart, .shop-traditional .products.ribbon li .buttons .add-to-cart:hover