How can I change background color of WHMCS shopping cart bar

With the order forms templates, you can't use custom.css - you need to directly edit style.css - in your case, standard_cart/style.css and change the CSS code below:

#order-standard_cart .view-cart-items-header {
    margin: 0;
    padding: 8px 15px;
    background-color: #058;
    color: #fff;
    font-size: 0.9em;
    border-radius: 4px 4px 0 0;