Bug 28927: (follow-up) remove rule on id=opacheader in opac CSS

Bug 28927: (follow-up) remove rule on id=opacheader in opac CSS

Actually this rule does nothing, it defines same background color as "body" : #fcf9fc

Test plan :
1) Without patch look at OPAC header
2) With patch OPAC header should have same background color

Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>