title: Aritchie userdialogs no background for ShowSuccess and ShowError on iOS
body: <p>I have installed Aritchie userdialogs version 5 and I'm using Xamarin.Forms Version 2.2.
  When I use UserDialogs.Instance.ShowSuccess or UserDialogs.Instance.ShowError, on Android a dark backgroud is displayed, on iOS is NOT displayed which makes this alert message pretty unreadable when the page behind is mostly white.
  On iOS I just get the message and the icon in the center of the screen but no dark background.</p>
  
  <p>Example code on iOS:
  UserDialogs.Instance.ShowSuccess("Data saved correctly");</p>
  
  <p>How can I solve it ?</p>