Hi there,
to add a border to the add to cart button requires some CSS:
.woocommerce ul.products li.product a.add_to_cart_button {
border: 1px solid #fff;
}
The button gets its colors from the Customizer > Colors > Buttons.
If you want to set the Add to Cart button to a different color ( not matching the theme styles ) then you can use this CSS instead ( it includes the border ):
/* Add to cart static color */
.woocommerce ul.products li.product a.add_to_cart_button {
border: 1px solid #fff;
background-color: #0F0;
color: #000;
}
/* Add to cart hover color */
.woocommerce ul.products li.product a.add_to_cart_button:hover {
border: 1px solid #fff;
background-color: #f00;
color: #fff;
}