userInterfaceStyle.swift

class ViewController: UIViewController {
    var userInterfaceStyle: UIUserInterfaceStyle?
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        userInterfaceStyle = self.traitCollection.userInterfaceStyle
        setupUI() 
    }

    
    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        //update user interface
        setupUI()
    }

    override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
        // Trait collection will change. Use this one so you know what the state is changing to.
        userInterfaceStyle = newCollection.userInterfaceStyle
        
    }

    private func setupUI() {
        switch userInterfaceStyle {
        case .dark:
            // User Interface is Dark
            ()
        case .light:
            // User Interface is Light
            ()
        case .unspecified:
            //your choice
            ()
        case .none:
            // Optional default case new introduction
            ()
        @unknown default:
            ()
            //Switch covers known cases, but 'UIUserInterfaceStyle' may have additional unknown values, possibly added in future versions
        }
    }
}