(Bug 5286) Light Gray Text Too Dark on Foundation Pages

Changes $_dark-gray to $_mid-gray and defines $_dark-gray as #636363
(as it is on a background of #e6e6e6), then assigns this color to
$sub-nav-font-color and $pagination-link-font-color and
$pagination-unavailable-link-font-color using intermediate
variables $sub-nav-color and $pagination-font-color.

Note: it appears secondary-color (mid-gray) is defined but never used.