[ticket/12598] Improve search box styling

PHPBB3-12598
[ticket/12598] Add lt IE9 fixes

In case this one gets merged before hanakin's gradient PR.

PHPBB3-12598
[ticket/12598] Remove unused link colors

PHPBB3-12598
[ticket/12598] Clean up unused link colors

PHPBB3-12598
[ticket/12598] Add some responsive margin to give breathing room

PHPBB3-12598
[ticket/12598] Remove hover border-color and use hover icon

PHPBB3-12598
[ticket/12598] Replace #search-box with class

PHPBB3-12598
[ticket/12598] Remove border from .search-header

PHPBB3-12598
[ticket/12598] Remove right border

PHPBB3-12598
[ticket/12598] Fix rtl positon in header

PHPBB3-12598
[ticket/12598] Fix sir typo

PHPBB3-12598
Merge remote-tracking branch 'hanakin/ticket/12598' into develop-ascraeus

* hanakin/ticket/12598:
  [ticket/12598] Fix sir typo
  [ticket/12598] Fix rtl positon in header
  [ticket/12598] Remove right border
  [ticket/12598] Remove border from .search-header
  [ticket/12598] Replace #search-box with class
  [ticket/12598] Remove hover border-color and use hover icon
  [ticket/12598] Add some responsive margin to give breathing room
  [ticket/12598] Clean up unused link colors
  [ticket/12598] Remove unused link colors
  [ticket/12598] Add lt IE9 fixes
  [ticket/12598] Improve search box styling