The quick-label CSS file is currently part of the Koha distribution, stored at koha-tmpl/intranet-tmpl/prog/en/css/spinelabel.css. This means that users who use package Koha cannot change the styling of the quick slips without resorting to javascript. I would suggest that koha-tmpl/intranet-tmpl/prog/en/css/spinelabel.css be a default, but that we can change this via syspref.
*** This bug has been marked as a duplicate of bug 16700 ***