Change BackgroundColor and alpha of UIView after UIViewController is presented

Question

A new UIView is displayed over the old one. While the UIViewController is presenting I want the background to be default. But as soon as it finishes presenting the UIView I want the background to turn lightGrey and the alpha should change from 0 to 0.6.

I know how to change the background and the alpha for my View, but how can I do these changes from the moment the UIViewController finished displaying?

Presenting UIViewController:

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
    let user = indexPath.row

    OperationQueue.main.addOperation {
        let storyboard = UIStoryboard(name: "Main", bundle: nil)
        let viewController = storyboard.instantiateViewController(withIdentifier :"ShowProfileView") as! showProfileViewController
        viewController.username = self.usernameList[user]
        self.present(viewController, animated: true)
    }
}

UIViewController:

class showProfileViewController: UIViewController {
    @IBOutlet weak var ViewController: UIView!
    @IBOutlet weak var profileLabel: UILabelX!
    var username = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        // Do any additional setup after loading the view.
        profileLabel.text = username
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        ViewController.layer.cornerRadius = 20
        ViewController.layer.masksToBounds = true
    }

    /*func updateProfileView() {
        view.backgroundColor = UIColor.lightGray.withAlphaComponent(0.6)
        view.isOpaque = false
    }*/        
}