In ViewDidAppear, I have changed the alpha value of the navigation bar to 0.15, but the right navigation bar item became almost invisible as well.
self.navigationController?.navigationBar.alpha = 0.15
You're setting the alpha of the entire bar to 0.15. You should be setting only the
barTintColor attribute of it to a color with an alpha of 0.15 like so:
self.navigationController?.navigationBar.barTintColor = UIColor.yourColorGoesHere().colorWithAlphaComponent(0.15)