To make it wider you will need to make the images wider in addition to css changes. It will be tricky to get it just right but it is possible to do it using photoshop. The image quality may degrade some though. I did the same thing on this site, the skin used on this site is based on styleshout-brightsideoflife but I made it wider. I had to do the same thing, make the images wider, and it was a tricky trial and error effort to get everything just right.
Hope it helps,