the easiest way to put in a css is to modify ad_header_with_extra_stuff so the head section looks something like
<head>
$extra_stuff_for_document_head
<title>$page_title</title>
<link Type="text/css" Rel=stylesheet HRef="/public/bf2.css">
</head>
("public" is a directory on my server that I've told ad_restrict_entire_server_to_registered_users to allow non-registered users access to.)