Unfortunately there is no option for changing the width of the theme and so this will be a customization you’d need to make. Because this is currently one of our older designs, it’s not made to responsive and so adjusting the width is going to take quite a bit of work to compensate for then all of the inner elements, as well.
For CSS changes, it’s a good idea to use a tool like firebug or Google Chrome’s built-in developer tools to locate them and then put the edits either in your child theme or in the “Custom CSS” option on your theme options page.
Also, here’s a helpful tutorial on how to use Firebug – http://vimeo.com/20917974. The only thing I’d suggest not doing is actually making edits to the theme’s CSS files, as it shows at the end of that video. However, the video serves as a good introduction to Firebug and how it can be used to find what CSS is effecting certain areas of your site without ever digging around in any files.