Hi,
The effect is provided by the template thanks to some CSS.
The "add to cart" button is placed on the image but is hidden by default ; when the mouse is hover the product, the CSS will display the button with a little effect.
See :
demo.hikashop.com/templates/jsn_glass_pr...jsn_ext_hikashop.css
.hikashop_products_listing_main .hikashop_subcontainer:hover .product-content .hikashop_product_stock .hikashop_cart_input_button {
visibility: hidden;
top: 38%;
opacity: 1;
filter: alpha(opacity=100);
-ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=100)";
visibility: visible;
}
After that, it will be difficult to tell you, we are not designers.
Regards,