The content area is the sibling of the overlay.
For the overlay to overlay it and its children it
must explicitly setting a z-index
additional changes:
* Move UI styles from skins.minerva.mainMenu.styles
to their more logical place (note skins.minerva.mainMenu.styles
is loaded via JavaScript unncessarily)
Bug: T193444
Depends-On: I7c0d02f073a2fe165a6027e3c6d1455e22c1b563
Change-Id: I8be31c7daf254e22ccaed81704cbcd2b638c54f6