For page types with an Advanced page-settings panel, open Edit, continue to Design, and use the Advanced icon under Page Settings. Enable Desktop Customization.

The panel exposes Left Image, Right Image, Left Color, and Right Color. Follow the upload limits shown by the editor and use colors that fit your page.

These options affect the desktop presentation around the central page. Review the public page on a desktop as well as a phone after saving. Not every page type exposes the same Advanced controls.