WP-Creativix Editing background image??

I have spent the last two hours searching through all of the templates on my edit page and I CANNOT find the spot in the CSS code where I can edit my background image. I would like to add this code:

#img.source-image {
width: 100%;
position: absolute;
top: 0;
left: 0;
}

In order to automatically size my background image. Help! Where do I find it??? My page is blogsussex.com