Add a custom stylesheet to checkout page.

add-custom-stylesheet-example.php

<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

// This is an example. You will need to modify the following to fit your specific site.

// Enqueue the stylesheet
add_action( 'wp_enqueue_scripts', function() {
  wp_enqueue_style( 'your-stylesheet-handle', $foo, $bar );
} );

// Add it to the allowed list of stylesheets for Checkout for WooCommerce
add_filter( 'cfw_allowed_style_handles', function($allowed_styles) {
	$allowed_styles[] = 'your-stylesheet-handle';
	return $allowed_styles;
} );