Location: Appearance > Customize > Global > Background
Applies to inside or outside of site container. To understand Site Container, refer to this link.
You can add both color and image as a background. There are following types of Background Options
Inside Container #
Adds background to the content inside the container of the Site.
NOTE: It appears to the full width for Wide Layout.
Outside Container #
Adds background to the content outside the container of the Site.
NOTE: This option will not work while Wide Site Layout as container will be full width.