I didn't see this anywhere else, but it seems the new admin theme clobbers the custom CSS that is in TinyMCE fields.
Without new admin theme:
With new admin theme:
Is this a known problem? Are there any solutions?
ok, i saw this comment
and have aded in the jquerycore file. this works for now, but I don't really understand why this file gets loaded for admin user but not other users. I can't see anything obvious in the Roles settings.