How do I add a Custom CSS File to the Checkout?

You can add a custom CSS file to the checkout by creating a checkout.css file in your public directory.

Example contents where we are changing the to use a black background:

.header {
    background-color:#000000;
}

.header__progress li:before {
    background-color:#ffffff;
    border-radius: 0;
}

.header__progress li.current:before {
    background-color:#ffffff;
}

.header__progress li.active {
    color: #00ea00;
}

.header__progress li.active:after, .header__progress li.active:before {
    background-color: #00ea00;
}

.header__progress li, .header__progress li.current {
    color:#ffffff;
}