[ticket/12740] Use class for logo in css

PHPBB3-12740
Merge pull request #2617 from hanakin/ticket/12740

[ticket/12740] Use .logo instead of #logo in css

* hanakin/ticket/12740:
  [ticket/12740] Use class for logo in css
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12740] Use class for logo in css