You can still customise the admin CSS. I suspect it just hasn't been
documented for the newer admin yet. But you'll see a bunch of .css files
in the django/contrib/admin/media/css/ directory and, if all else fails,
poking around with something like Firebug will show the relevant classes
fairly quickly.
If you are playing around in that area, take notes as you go and you
might be able to help us resurrect the old documentation and make it
appropriate for the new admin.
Regards,
Malcolm