gitweb: Use light/dark for class names also in 'blame' view

gitweb: Use light/dark for class names also in 'blame' view

Instead of using "light2" and "dark2" for class names in 'blame' view
(in place of "light" and "dark" classes in other places) to avoid
changing style on hover in 'blame' view while doing it for other views
(like 'shortlog'), use more advanced CSS, relying on the fact that
more specific selector wins.

While at it add a few comments to gitweb CSS file, and consolidate
some repeated info.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>