I was just working on something like this today. I used a template hook. I was working on something for my members page, so I used the hook location "member start" to add some css like this:
PHP Code:
$template_hook[memberinfo_css].= '
#fakecss fieldset.fakecss_group {
margin-top:6px;
}
fieldset.fakecss_group table {
margin:6px;
}';
It goes into the page, not the external.css file though.