Theme content area width

Hi. I need help. I manage over 50 wordpress websites. I use a variety of themes. One thing all themes have in common is the content area is too narrow leaving too much space on both sides. Please see rattrapping.biz as a quick example. The gray wood on both side is too much wasted space. Is there a generic css code that I can add that will widen the content area BUT to also shift the sidebar accordingly. It seems that most themes are created this way. Please advise the easiest way to accomplish this.