Because the "Cart" is a part of your theme, those settings can be customized through the theme editor under Theme settings > Cart.
Once your visitor selects "Checkout" they have transitioned from the "Cart" to "Checkout" which is no longer part of your theme and is controlled by Shopify.
In the Checkout, Shopify does allow for some basic style, color, and image adjustments that are universal across all themes. You can edit the options found under Theme Settings > Checkout or follow their guide on customizing the style of your checkout here.
One exception is that merchants on Shopify's top-tier Plus plan have access to further modify their checkout through a custom checkout.liquid asset as described in Shopify's developer documentation.