Themeable ViewController demonstration

Themeable.swift

import UIKit

protocol Themeable {
	func applyTheme()
}

extension Themeable where Self: UIViewController {
	func applyTheme() {
		// beautiful!
		navigationItem.leftBarButtonItem?.tintColor = .purpleColor()
	}
}

class ThemedViewController: UIViewController, Themeable {
	override func viewDidLoad() {
		super.viewDidLoad()
		applyTheme()
	}
}