Load.php CSS

I'm having a hard time trying to figure out where this css is coming from. I'm using the default mono book skin.

<link rel="stylesheet" href="http://www.site.com/w/load.php?debug=false&lang=en&modules=site&only=styles&skin=monobook&*" type="text/css" media="all" />

Where is the actual css file. It's not main.css in the monobook folder because that has different css attributes.

You don't mention which version of MW you have, but I'll assume it is a recent one. Put &debug=true at the end of the url you use to load the page. That should tell ResourceLoader to load each style sheet separately and make it a little easier to track down.