XSL-FO and CSS stylesheets

Hi List.
I was wondering how one can specify the style to be used when going from
docBook to XSLFO (then PDF). I mean, when going to HTML, you can specify
a CSS file that will style the output nicely, but how can one do that
with xslfo ?
The idea would be to generate the same output "look" with HTML and PDF
(which seems to be so common something should exist...)

Thanks in advance,
Regards,
Fabien