Swift change font and color of back button

Swift 3.0 answer (based on Lion's answer):

let newFont = UIFont(name: "Avenir Next", size: 16.0)!
let color = UIColor.white

UIBarButtonItem.appearance(whenContainedInInstancesOf: [UINavigationBar.classForCoder() as! UIAppearanceContainer.Type]).setTitleTextAttributes([NSForegroundColorAttributeName: color, NSFontAttributeName: newFont], for: .normal)

Works a treat, for those that have already managed to customise other parts of their nav bars but not the back button!

If you want to set same color to bar buttons implicitly then in your AppDelegate, in didfinishlaunchingwithoptions, write:

 UINavigationBar.appearance().tintColor = UIColor.white //your desired color here

Update :

Put this in AppDelegate,

 UIBarButtonItem.appearance().setTitleTextAttributes([NSFontAttributeName: UIFont(name: "Andes Rounded", size: 17)!], forState: .Normal) // your textattributes here

Update 2 :

  UIBarButtonItem.appearanceWhenContainedInInstancesOfClasses([UINavigationBar.classForCoder()]).setTitleTextAttributes(["attribute" : "value"], forState: .Normal)

Hope this will help :)