This is a sample that can be reused to change the font / colours / tint of your navigation bar.

Swift 3 - Appdelegate NavigationBar Customisation

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        
        // Override point for customization after application launch.
        UIApplication.shared.statusBarStyle = UIStatusBarStyle.lightContent
        
        let navigationController : UINavigationController = self.window?.rootViewController as! UINavigationController
        
        // Change the navigation bar background color to blue.
        navigationController.navigationBar.barTintColor = UIColor(red: 243/255, green: 129/255, blue: 37/255, alpha: 0)
        
        // Change the color of the navigation bar button items to white.
        navigationController.navigationBar.tintColor = UIColor.white
        
        let font : UIFont = UIFont (name: "Avenir Next Medium", size: 14)! // Unwrapped
        let colour = UIColor.white
        let attributes: [String : AnyObject] = [NSFontAttributeName: font, NSForegroundColorAttributeName: colour]
        navigationController.navigationBar.titleTextAttributes = attributes
        
        let font2 : UIFont = UIFont (name: "Avenir Next Medium", size: 16)! // Unwrapped
        let colour2 = UIColor.white
        let attributes2: [String : AnyObject] = [NSFontAttributeName: font2, NSForegroundColorAttributeName: colour2]
        UIBarButtonItem.appearance().setTitleTextAttributes(attributes2, for: UIControlState())
        
        
        return true
    }