Merging background with the posts and sidebar containers

Hi There
I hope I will make sense.
I would like to achieve something similar to these website http://simplemom.net/ and http://zenfullydelicious.com/
I would like the background to merge with the posts and sidebar containers and I would like to have the illusion of a ribbon going across the whole width of the screen aligned with with the category and page menu. I hope it makes sense.
Is there a way to achieve this with Atahualpa?
Many thanks in advance,
Peggy