Done. Perfect, thanks.
I looked at the assignment location, but decided to assign after the 'issset($journal)', where the custom journal style sheet is picked up. Though now that I think about it, perhaps it would be better to put it outside that block so that it would be picked up when there are no journals and an administrative user is logged into OJS itself, and not necessarily associated with a site. Actually, I can imagine two of these files, one for OJS and another per site. I don't need anything that advanced at the moment.
A CSS file applied when a user is logged in makes it quick to do things such as change colours to remind people they are logged in (and need to log out), or change layout, etc.
Although perhaps these changes should be done in the generated HTML (i.e. not even sending the elements to the browser in the first place). Only purely cosmetic changes should be in the stylesheet obviously.
Is it worth me cleaning this up as plugin, or will the coming layout tools provide this type of functionality? (I pick up the file from the journal's public files section -- cutting and pasting the code from the existing stylesheet detection, so I could add a journal setting to check and avoid hardcoding a path, uploading, etc. etc.).