Hello,

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

.cart-collaterals .cart_totals .wc-proceed-to-checkout>a.button,
.cart-collaterals .cart_totals .wc-proceed-to-checkout>a.button:hover,
.cart-collaterals .cart_totals .wc-proceed-to-checkout>a.button:focus{
    background-color: #000000;
    border-color: #000000;
}

Best Regards