[Theme: Stockholm] adding margins

The stockholm theme is currently full page, I would like to carry on using it but add margins so my content is centered and looks more filling on the page. Kind of like this http://camendesign.com/code/developpeurs_sans_frontieres

I dont want any content in the margin space, just the page divided up 20/60/20 below the title bar. Is there CSS code that could achieve this?