Check out example codes for "dark mode change immediately swift". It will help you in understanding the concepts better.

Code Example 1

guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else {
   return
}
appDelegate.changeTheme(themeVal)


// App Delegate Change Theme method
func changeTheme(themeVal: String) {  
  if #available(iOS 13.0, *) {
     switch themeVal {
     case "dark":
         window?.overrideUserInterfaceStyle = .dark
         break
     case "light":
         window?.overrideUserInterfaceStyle = .light
         break
     default:
         window?.overrideUserInterfaceStyle = .unspecified
     }
  }
}

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