It seems ForumWP plugin is breaking the Contextual Adminbar Color behavior. This is a plugin used to customize Admin Bar color (very usefull to manage dev/ preproduction / production environnements).
ForumWP seems to break it as it deny the correct loading of CSS rules from the other plugin. Apparently the body of wp-admin pages don't get the "contextual-adminbar-color" class added. ForumWP is the only plugin that break the behavior of Contextual Adminbar Color plugin as far as I know.
Do you have any clue why ?