AppDelegate.swift

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
  
  // Initialize the window
  window = UIWindow.init(frame: UIScreen.mainScreen().bounds)
  
  // Set Background Color of window
  window?.backgroundColor = UIColor.whiteColor()
  
  // Allocate memory for an instance of the 'MainViewController' class
  let mainViewController = MainViewController()
  
  // Set the root view controller of the app's window
  window!.rootViewController = mainViewController
  
  // Make the window visible
  window!.makeKeyAndVisible()
  
  return true
}  

Hi,

if you like to update the nice tutorial to Swift 3:
Line 4: window = UIWindow.init(frame: UIScreen.main.bounds)
Line 7: window?.backgroundColor = UIColor.white

Cheers