[ticket/12811] Fix padding/margin bug affecting bottom layout

PHPBB3-12811
[ticket/12811] Responsive adjustments to margin changes

PHPBB3-12811
Merge remote-tracking branch 'VSEphpbb/ticket/12811' into develop-ascraeus

* VSEphpbb/ticket/12811:
  [ticket/12811] Responsive adjustments to margin changes
  [ticket/12811] Fix padding/margin bug affecting bottom layout