Editing header

This is a really interesting Theme, which I think will do everything I want, but I do want to change the overall look to match my existing site a bit more.

Starting at the top, I’m wondering how to edit the background colour of the #header id. Looking at the source, there is a section like this:

#header{ background:#505050 url(); }
#header #site-heading a, #header #site-description { color:#ffffff; }
#header #site-heading a:hover, #header #site-heading a:active { border-bottom:#ffffff 1px dotted; }

This is overriding the setting in mystyle.css, but I cannot see how this is getting into the page.

I looked in header.php but I cannot see anything like this.

Any suggestions?