Hi Joe,
I've noticed that those persons in a role assigned to the Site Editors permission do not have access to the custom css class settings on a page. Site Editors are supposed to be equivalent to Content Administrators for a particular site when using Related Sites Mode, correct? Adding the same role to the "Roles that manage skins" permission alleviates the problem but it seems like an extra step. If not, we just need to document it. :-)
Thanks,
Joe