Click the color swatch to set the color of the background.
Use custom Background
Check this option to enable the facility to upload an image (1000 x 400px recommended) to be used as a custom background for the checkout.
Select Custom from the dropdown field to enable the facility to upload an image (250 x 100px recommended) to be used as a custom logo for the checkout.
Optimized Checkout Appearance
Using the other options in this panel you can configure the font sizes and text, link and icon colors that will be used in the Optimized Checkout.
For more information about the Optimized One Page Checkout system, refer to the following BigCommerce support article.