You can use Custom CSS code, for example:
http://prntscr.com/o9znpz
https://fontawesome.com/v4.7.0/icons/
.product-view-mask3 .footer-product>.wcmp_add_to_cart_message:before {
content: "\f017";
font-family: FontAwesome;
font-size: 19px !important;
border-color: #444444;
background: #222222;
color: white;
border-radius: 50px;
width: 35px;
line-height: 2;
}
.product-view-mask3 .footer-product>.wcmp_add_to_cart_message {
font-size: 0 !important;
}
change this code depends on your needs.
Regards