Declaring variable to keep UIViewController that conforms to protocol Themeable

ThemableViewController.swift

protocol Themeable {
    var viewController: UIViewController { get }
}

extension Themeable where Self: UIViewController {
    var viewController: UIViewController { return self }
}

class ThemeableViewController: UIViewController, Themeable {}
let themeable: Themeable = ThemeableViewController()

// accessing viewController
themeable.viewController.view.backgroundColor = UIColor.blackColor()

class ThemeableObject: NSObject, Themeable {} // error

For more information,
http://qiita.com/codelynx/items/30997a4c6f685af4fabc
(In Japanese)