To change background color on a CEE site, edit the inst.css file directly.
Add this line exactly as it appears: body { background-color:#ffff99 to the bottom of this file (if it exists) in the 'look' project: /look/www/overrides/static/csi/css/inst.css If inst.css file does not exist, it can be created by going to http://[domain.com]/branding/css/inst.css and using that file content to create inst.css in the following location in the 'look' project: /look/www/overrides/static/csi/css/inst.css. Any additions/changes should be appended to that content. (issue 39881)
If site has a stylesheet i.e. stylesheet in the 'look' project, i.e. /look/www/overrides/static/csi/css/styles.css, then this page will get parsed after inst.css and can override the site's colors. In this case, add the background-color setting to the bottom of styles.css instead.
Color codes are available from a number of sources, including the link shown here.