Try adding this
.woocommerce a.button, .woocommerce button.button, .woocommerce input.button, .woocommerce #respond input#submit, .woocommerce #content input.button, .woocommerce-page a.button, .woocommerce-page button.button, .woocommerce-page input.button, .woocommerce-page #respond input#submit, .woocommerce-page #content input.button {
text-shadow: none;
}
.woocommerce a.button:hover, .woocommerce button.button:hover, .woocommerce input.button:hover, .woocommerce #respond input#submit:hover, .woocommerce #content input.button:hover, .woocommerce-page a.button:hover, .woocommerce-page button.button:hover, .woocommerce-page input.button:hover, .woocommerce-page #respond input#submit:hover, .woocommerce-page #content input.button:hover {
background: none repeat scroll 0 0 rgba(0, 0, 0, 0);
text-decoration: none;
}
It may cause color issues for other buttons, if it does please let me know
– Kyle