title: Run a function located in one UIViewController from a different controller
body: <p>I currently have two UIViewControllers, a Calculation UIViewController, and a Settings UIViewController. I'm attempting to make a dark and light theme. In the Calculation VC, I have a theme function:</p>
  
  <pre><code> func setCandyTheme() {
      print("Theme enabled!")
      configureGradientBackground(UIColor(red: 0.14, green: 0.85, blue: 1.0, alpha: 0.8).CGColor, UIColor(red: 0.24, green: 0.65, blue: 1.0, alpha: 0.8).CGColor)
      for view: UIView in containerViews {
          view.backgroundColor = UIColor(red: 1, green: 1, blue: 1, alpha: 0.18)
      }
      for textField: UITextField in textFields {
          textField.backgroundColor = UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 0.20)
      }
      for textField: UITextField in weightTextFields {
          textField.backgroundColor = UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 0.25)
      }
  
  }
  </code></pre>
  
  <p>In the Settings VC, I made a toggle for enabling the theme. When it's enabled, it sets the themeKey value (which works) and runs the setCandyTheme function.</p>
  
  <pre><code> func switchIsChanged(themeToggle: UISwitch) {
  
      let storyBoard = UIStoryboard(name: "Main", bundle: nil)
      let calcViewController = storyBoard.instantiateViewControllerWithIdentifier("Calculate") as! ViewController
  
      if themeToggle.on {
          defaults.setValue("True", forKey: themeKey.key)
          calcViewController.setCandyTheme()
      }
      else {
          defaults.setValue("False", forKey: themeKey.key)
      }
  
  }
  </code></pre>
  
  <p>Unfortunately, the theme isn't applied but the <code>"Theme enabled!"</code> string is printed in the console. I assume it's running the function in the Settings VC rather than Calculation, which is why it's not being applied. </p>
  
  <p>What's the solution for this? I've been at it for hours and any help would be much appreciated!</p>