[ticket/12292] Improve style details page

http://tracker.phpbb.com/browse/PHPBB3-12292
Gave style details page a proper heading and back button.
Also removed a few obsolete code leftovers in the style
and CSS.

PHPBB3-12292
Merge remote-tracking branch 'vsephpbb/ticket/12292' into develop-ascraeus

* vsephpbb/ticket/12292:
  [ticket/12292] Improve style details page
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12292] Improve style details page
  [ticket/8309] Changing $last_activity to $last_active
  [ticket/8309] Removing key with similar function to avoid redundancy
  [ticket/8309] Moving key to get keys in alphabetical order
  [ticket/8309] Rename $last_visit to $last_activity
  [ticket/8309] Renaming "Last visit" to "Last activity"