You can expand the width of your content area by increasing the width in this part of the style sheet:
background: url("images/bg_container_top.jpg") no-repeat scroll center top transparent;
padding: 0 0 16px;
However, since the image is set to 1000px, this will not look the way you probably want it to.
I looked at the theme as well as the example site you linked to using the Firebug extension with Firefox. It looks like on the example site that they took the background image for the container and spliced it up so that they could expand the width and use the pieces of the original image. I would recommend taking a look at their style sheet and using the Firebug extension to pinpoint each part of the site so you can see what CSS controls the different parts of the site.
Also, before you make changes to the theme, make sure you are using a Child Theme so future updates to the theme do not overwrite your changes.
Hope this helps!