Update popup-imagemanager.css

update popup-imagemanager.css for the following tracker:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=30947

PS: is there are some less files there must be updatet to?
Merge pull request #1151 from zero-24/patch-6

Update popup-imagemanager.css