The Shopify checkout area is the screen your customers see once they've clicked the Checkout button from their cart. This area is completely independent of the theme and is fully controlled by Shopify. Therefore, it is not possible to make layout or functional changes to this page unless you are a Shopify Plus customer. You can, however, make a few minor styling changes, such as changing fonts, colors, buttons, and adding your own logo and background image.
For more information about how to customize your checkout page, please see this Shopify help article.