Without seeing your site, who knows….
Use Firefox & get the firebug plugin, it generally makes things a lot easier.
In your case though it looks like that theme is based on the bluetrip css framework (or some variant), changing the width isn’t quite as easy as with other themes. You’d need to change the width of the wrap & container divs, as well as all of the span-1, span-2 (& so on) up to span-24 divs, as they’re all fixed widths based around a 24-column layout & 980px overall width.
You’d need to decide on a new overall width (that’s divisable by 24 plus the width taken up by the in-built margins), & change the span-1 to 24 divs accordingly, as well as the divs that control the overall width. Using fluid width with bluetrip is pretty much not going to happen though if that’s what you want.
