How to transparent background when pushViewcontroller

Question

how to transparent background when pushViewcontroller?? just like.. modal view over full screen (programmatically, not using storyboard, not using present modal)

---

first view controller > Button Action >

    let transition = CATransition()
    transition.duration = 0.5
    transition.timingFunction = CAMediaTimingFunction(name: kCAMediaTimingFunctionEaseInEaseOut)
    transition.type =  kCATransitionPush //kCATransitionFade
    transition.subtype = kCATransitionFade

    self.navigationController?.view.layer.add(transition, forKey: nil)

    let testVC = SecondViewController()

    if let existVC = self.navigationController {
        existVC.pushViewController(testVC, animated: false)
    }

---

second View Controller

super.viewDidLoad()
self.view.backgroundColor = UIColor.clear

The result is... black background. how to transparent background??