Editing a page with site builder
Design and Skin Customization
Site builder system is a content construction system, not a skin stylizing system. To change the background color of the skin you will need to either edit the CSS files in the skin, or use the STYLE tab in the ACP.
See these docs for skin designing tips:
Docs: "Skin Customization: Table of Contents"
https://www.jamroom.net/the-jamroom-network/documentation/skin-customization