Top bar link hover background color was being ignored due to small SCSS error. This fix causes the hover link color and background variables to be respected since they're being applied in the correct selector.
Added SCSS for Top Bar to degrade gracefully when Javascript is disabled.

When Javascript is disabled, allows dropdowns to appear on hover, and
also allows top bar menu items' background and font color to change on
hover.
Merge pull request #2483 from prembo/master

Fix enabling top bar link & background colours (set via variables in _settings.scss) to have effect