Update CSS for header

[bebaps]

Currently Nifty Nav uses this code

header{
  z-index: $header-index;
}

but this should probably be namespaced in some way. In its current state, it is applying this z-index to any header in the site, and this can potentially cause unexpected issues in other areas, for example any header that may be placed inside on an article, main, etc.

[erwstout]

I agree this should be adjusted.

[erwstout]

This was changed in release 3.0.0