Adding white space to left and right of 960 grid

I hate to start new threads when not necessary, but stumped once again.
I am using a theme built on the 960 grid. I like it good enough, but would like to a background color while keeping the parent container white.

Problem is the default margins on the left and right of the container are too small: 10 px. So not getting enough white space for my liking. Is there a way of adding more white space to the left and right of .container_12 while not disrupting the rest of the CSS? I don’t want to rewrite more than necessary for fear of having to rename my CSS and corresponding template files.

Here is my site: http://www.mattlorenzi.ca