Hi,
You indeed need to add custom CSS in the frontend CSS of HikaShop for that.
The exact CSS will depend on what you want.
On our demo website, we added:
.hikashop_product_quantity_main .hikashop_product_stock .button {
color: #167e93 !important;
font-weight: bold;
background-image: url(../images/add_cart.png);
background-repeat: no-repeat;
background-position: right;
height: 32px;
padding-right: 40px !important;
background-color: transparent;
cursor: pointer;
}
So first, it's normal that you don't see .hikashop_product_quantity_main .hikashop_product_stock .button in the default CSS, since you need to add it

Second, you changed the button display from "normal" to "css", and thus you should use .hikashop_product_quantity_main .hikashop_product_stock a.hikashop_cart_button instead of .hikashop_product_quantity_main .hikashop_product_stock .button to customize the CSS of the add to cart button.