Access container view child properties swift

Question

What I want to achieve:

User presses the button in the ViewController then, the color of the button placed in the container view should change its color to red.

Answer 1

Swift 4, Xcode 9.4.1

var contentViewController : UIContentViewController?

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == containerSegueName {
       contentViewController = segue.destination as? UIContentViewController
    }
}