branding: Do not require style sheet files

- Removed requirement that style sheet files must be defined in
  branding.properties.
- Fixed issue where if a welcome page template would access a key
  that wasn't defined in the properties file it would cause a npe.

Change-Id: I6e2b60060557ce15b255d5f30b216d5715b3d095
Signed-off-by: Alexander Wels <awels@redhat.com>