You can find the Layouts option in the theme Customizer on Appearance → Customize → Layouts -> Site Container. The theme supports several features you know:
Site Full-Width Layout
The full-width layout displays your content centered, but with a white background throughout. This layout doesn’t have any padding around the main wrapper.
Site Boxed Layout
The boxed layout has a padding around both sides of the main wrapper as well as a background applied to it so that you can apply a custom background to your body and all your content will be centered with the background flowing around it.