ThemeManager-DemoViewController.swift

import UIKit

class DemoViewController: UIViewController {
    @IBOutlet weak var highlightedLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Subscribe this instance of UIViewController to theme notifications
        ThemeManager.shared.register(
            target: self,
            action: DemoViewController.applyTheme)

        // Subscribe `highlightedLabel` instance of UILabel to theme notifications
        ThemeManager.shared.register(
            target: highlightedLabel,
            action: UILabel.applyHighlightedLabelTheme)
    }
}

extension DemoViewController {
    /// Handles theme notifications receiving a new theme.
    fileprivate func applyTheme(_ theme: MyTheme) {
        view.backgroundColor = theme.settings.appBgColor
    }
}

extension UILabel {
    /// Handles theme notifications receiving a new theme.
    fileprivate func applyHighlightedLabelTheme(_ theme: MyTheme) {
        switch theme {
        case .light:
            text = "Light Mode"
        case .dark:
            text = "Dark Mode"
        }
        textColor = theme.settings.textColor
        backgroundColor = theme.settings.highlightedBgColor
        sizeToFit()
    }
}