[HowTo] Transparent SubViewController

@objc func addButtonTapped() {
    let nav = //AddCityViewController()
    nav.modalPresentationStyle = .overFullScreen
    self.present(nav, animated: false, completion: nil)
}

//AddCityViewController

override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = UIColor.init(white: 0.3, alpha: 0.4)  
}

Github Link

Transparent SubViewController Example Github Link (opens new window)