Following points might be helpful to you.
Grey background permeates the whole website, not just the fringe.
I think you are talking for Wide Layout. Just follow the steps,
- Go to Appearance > Contango Options
- Select Layout Settings tab
- Choose Wide Layout and Save Changes
Now you are in Wide Layout mode, Now you can beautify or adjust it according to your taste by applying,
- Suitable Texture by going to Appearance > Background
- Upload your High Retina Logo i.e 560 x 150 px via Appearance > Header