This theme is based on the popular 960.gs CSS Grid Framework.
You can easily adjust the theme's layout settings by modifying the width of both the left and right column from within the Joomla Administrator section. The content column in the center automatically adjusts according the the available width.
These are available under Extensions → Template Manager → [Template]. You can choose to set the position of each of the columns in the template under the template options, click on the images above to see the layouts in action. Additionally, you can also choose to set the width of each column or hide any of the columns.