I would first recommend upgrading your CMS to the Latest version (1.8.11) to see if the issue is resolved. If you are still experiencing the same problem after the upgrade, please let me know.
I just upgraded on a clone system , and using "body { … " on css style still does not work, BUT, it’s probably because you can use the background colour picker on the last tab.
I’m closing as not a bug, pebkac related.