Add $topbar-link-hover-bg, instead of just being black.

I have to override the standard black color with the following rule:

.top-bar-section li a:not(.button):hover {
  background-color: $topbar-menu-link-color-toggled;
}

It would be great if that was a variable set instead, so I didn't have to have that mess in my stylesheet.

I'd love to submit a PR on that if it's something you'd be interested in.