Fix for two errors in _sub-nav.scss

1. sub-nav-active-bg-color was hard-coded. Now it is set to proper value
from _settings.scss

2. the hover color for the sub-nav-font was erroneously set to a
background color. Now it is set to the proper value from _settings.scss.
Merge pull request #4113 from karland/patch01

Fix for two errors in _sub-nav.scss