[ticket/12821] Convert remaining gradients to CSS

PHPBB3-12821
[ticket/12821] Fix current gradients for consistency

PHPBB3-12821
[ticket/12821] Bump tweaks.css to IE9

PHPBB3-12821
[ticket/12821] Remove all round corners in IE9

- Make IE9 look like IE8 due to inability to handle round corners with css  gradient backgrounds

PHPBB3-12821