Editing of the spinelabel.css file should be done via a system preference as this is thought to be library specific. The spinelabel.css file is used for formatting labels printed with the quick spine label creator.
I have now figured out that you can use intranetusercss and soon intranetuserjs on the page. Maybe the extra CSS file should be removed? It can't be edited by libraries and will be overwritten during updates.
I think it's good to have default styling always in place. intranetusercss can be used to override whatever is in spinelabel.css. In my opinion this bug would be WONTFIX. On the other hand, we could add a new system preference which was specifically for spine label CSS. If that's what is wanted the title of the bug should be revised.
Agreed :)