Hello,

Please add this code to the Theme Settings > Custom CSS > Global:

body .woocommerce-mini-cart__buttons .btn-cart {
    color: white;
    background-color: red;
}
body .woocommerce-mini-cart__buttons .btn-cart:hover {
    color: white;
    background-color: green;
}

Set the colors as per your needs.

Best Regards