Background image only on homepage

I'm working with Omeka v.2 using Berlin theme. How would I add a background image only to the homepage? Sorry this seems like such a simple question but i couldn't find anything on this in the forum.

The theme file is index.php at the top of the theme's directory. You could add CSS to just that page by including some CSS via the queue_css_file() or queue_css_string() functions, before the call to head()