Modularity Lite Page Width Limiter?

Hi all,
I’m new to wordpress editing and I’ve learned a lot from pouring over the forums here. It has been a guess-and-check learning process though.

I’m developing the site for the company I work for http://www.atlasstageworks.com and I’ve come to an issue with the overall width of the background. If you extend the width of the browser, eventually you overshoot the background image I’ve uploaded and it begins to show the default blue.
Is there a way to limit how wide the page can be, or even to unlock the background aspect ratio to stretch if the page becomes wider? I ultimately wish to not have a blue border show up regardless of how wide the browser is.

Thanks for your help.