This is a feature of graphene 1.6 which shows the actual content area of your page. This is in an attempt to help the visual editor become more wysiwyg friendly. If you expand the content section in the custom columns, the grey in the editor (on the sides) will adjust also. As you enter text, the editor will “grow” vertically.
As we all know, the visual editor is far from what you see in the editor is what you see on your main page.
However, attempts are being made to get these two items to more closely resemble each other.