This is the default behaviour of that background functionality, as this styles the edited line. Page styling is usually made in the external CSS file.
In order to facilitate the editors work, you could add dedicated styles for Footer, Header, etc, in your main.css file and to ask the Editors just to add a specific class in the Content Block. For example cbFooter, cbHeader, etc. Thus the ContentBlock will be styled "automatically".
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items