At the moment it is possible to change it only via CSS

.single-product-content .cart .button {
    background-color: #81b441;
    color: #fff;
    border-color: #81b441;
}
.single-product-content .cart .button:hover,
.single-product-content .cart .button:focus {
    background-color: #70a232;
    color: #fff;
    border-color: #70a232;
}

Add it to the Custom CSS section in Theme Settings