[ticket/12821] Convert .headerbar to css gradient

PHPBB3-12821
[ticket/12821] Convert .forabg to css gradient

PHPBB3-12821
[ticket/12821] Convert .forumbg to css gradient

PHPBB3-12821
[ticket/12821] Convert .forums to css gradient

PHPBB3-12821
[ticket/12821] Convert .button1-3 to css gradient

PHPBB3-12821
[ticket/12821] Fix .button gradient for consistency

PHPBB3-12821
[ticket/12821] Fix .activetab gradient for consistency

PHPBB3-12821
[ticket/12821] Fix #navigation gradient for consistency

PHPBB3-12821
[ticket/12821] Fix dropdown gradient for consistency

PHPBB3-12821
[ticket/12821] Do not repeat bg on .button1-3

PHPBB3-12821
[ticket/12821] Remove obsolete images

PHPBB3-12821