Your page source shows:

Code:
<link rel="stylesheet" type="text/css" href="includes/templates/CP/css/stylesheet.css" />
<link rel="stylesheet" type="text/css" href="includes/templates/CP/css/stylesheet_categories_menu.css" />
<link rel="stylesheet" type="text/css" href="includes/templates/CP/css/stylesheet_css_buttons.css" />

<link rel="stylesheet" type="text/css" media="print" href="includes/templates/CP/css/print_stylesheet.css" />
So your CSS files are all in:

includes/templates/CP/css/