Content box in the middle

Hey, I’d like to have a main box where the text from every site gets into.

I have removed all sidebars so now i have the full width. But I’d like to have the width of as if I had 1 sidebar at the left and 1 sidebar at the right.
A background-picture should not be the background of my main-content-box. Only of the rest space at the right and left.

http://www.change-dein-leben.de/produkte/ab/die-gluecksformel-ueberraschung/

look here what i mean with the box in the middle. Thats at least how it should look like (kind of) I’m using Customizr

Thanks for any response