How to increase overall width of theme?

Hi all, Got someone who wants to increase width of the theme overall. I'm not talking about the primary and secondary column widths. I'm actually talking about the section that's colored #EEEEEE that surrounds the primary and secondary columns. I've cautioned him about variable monitor widths/resolutions. But, he still wants to try to get some more real estate on the screen. Where is that specified in the theme? Thanks!! Robert