User blog comment:Rupert Giles/Global Navigation January Update/@comment-5510093-20150130182455/@comment-24416108-20150131134147

Higher Opacity to accommodate lighter wiki's will hinder the dark ones again.

Other ways could be to have it go transparent:
 * in the wiki's own background color
 * in a color to be set in the theme designer
 * auto-adaptive to the light/dark scale of the wiki in question.