The Storefront Checkout Customizer extension gives you additional style and layout options for the WooCommerce sections of your Storefront powered website.
This extension integrates with the WordPress Customizer making it extremely simple to use. Once you enter the Customizer you will notice a new top level section: Checkout. The Storefront Checkout Customizer settings menu item will only display if you are viewing the WooCommerce Checkout page from within the customizer and the settings should be self explanatory.