Fix for stray 1px line under top navigation

If you add an "active" class to one of the <li> elements in the top navigation, and style it so that active tabs have a different color, it becomes clear that there is a 1 pixel stray line under the navigation. This style fixes that.
Merge pull request #12897 from avindra/patch-1

Fix for stray 1px line under top navigation in Dashboard example