Change header area to span the entire page?

Hi everyone! Loving the Catch Box theme. I have some background in creating sites from scratch using HTML5 and CSS3 and did make a theme from scratch for Tumblr. Editing WordPress themes in depth though is new to me. I wanted to change the header for this theme to span across the entire page, then leave the areas below as-is. The way I've done this in the past is to keep the header outside of the container that the aside and section lives in, and I altered this in the index page with the HTML. However, in WP and for this template, the index.php does not show these tags. So, I'm not quite sure where I go in order to make this change. Feeling a bit lost! This is my site: geekpinata.com Thanks for any help!