[ticket/12378] Prosilver common.css duplicate entry

https://tracker.phpbb.com/browse/PHPBB3-12378

PHPBB3-12378
Merge pull request #2402 from MGaetan89/ticket/12378

[ticket/12378] Prosilver common.css duplicate entry

* MGaetan89/ticket/12378:
  [ticket/12378] Prosilver common.css duplicate entry