DarkMode.swift

class AppDelegate {
    func appDidFinishLaunching() {
        //Before the app shows root view controller, check for an override style.
        window = UIWindow()
        if let override = UserDefaults.standard.bool(forKey: "OverrideStyle") {
            //There was an override set, use that otherwise use the system appearance
            window?.overrideUserInterfaceStyle = override ? .dark : .light
        }
    }
}

extension UIColor {
    static let foregroundPrimary = UIColor { trait in
        //bad color choices but this is to illustrate the UIColor with trait block.
        if trait.userInterfaceStyle == .dark {
            return UIColor.red
        }
        return UIColor.blue
    }
}

class MyViewController : UIViewController {
    
    let label = UILabel()
    let layer = CALayer()
    
    func viewDidLoad() {
        super.viewDidLoad()
        //This color will automatically change when stlye changes.
        label.textColor = UIColor.foregroundPrimary

        //layers require CGColor, so they have to be updated on trait change.
        layer.backgroundColor = UIColor.foregroundPrimary.cgColor
    }

    override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        //layer colors don't automatically change, so you need to update in trait collection change.
        layer.backgroundColor = UIColor.foregroundPrimary.cgColor
    }

    func onDark() {
        //true assumed to be dark
        UserDefaults.standard.set(true, forKey: "OverrideStyle")
        window?.overrideUserInterfaceStyle = .dark
    }

    func onLight() {
        //false assume to be light.
        UserDefaults.standard.set(false, forKey: "OverrideStyle")
        window?.overrideUserInterfaceStyle = .light
    }

    func onClearStyle() {
        //clear to use the system defined appearance.
        UserDefaults.standard.removeObject(forKey: "OverrideStyle")
    }
}