[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 conssitency

PHPBB3-12821
[ticket\12821] Fix #tabs Gradient for conssitency

PHPBB3-12821
[ticket\12821] Fix #navigation Gradient for conssitency

PHPBB3-12821
[ticket\12821] Fix .dropdown-extended .header Gradient for conssitency

PHPBB3-12821