Optimized Checkout (Portobello)
Configure the appearance of the Optimized Checkout. In your control panel, go to Storefront > My Themes, click the blue "Customize" button to launch Theme Editor, then select the "Theme Styles" icon. In the left hand column of the Theme Editor, click the panel labelled 'Optimized Checkout'
You can change the colors of elements in the Optimized Checkout and Order Confimation pages in this section.
For more information about the Optimized One Page Checkout system, refer to the following BigCommerce support article.