title: React Navigation transitionConfig Color
body: <p>I see there is an option for transitionConfig to enter values such as the animation style or the duration. I was wondering if there was a way to change the color of the background during the transition (the semi-transparent background that appears during the transition)? For example, I have fairly dark screens and during the transition the background kind of flashes white.
  Is this  either configurable directly, or is it maybe a property of the parent navigator?
  Thanks</p>
  
  <p>I see during the transition process the area between the focussed screen and the edge of the screen goes from white with an opacity of 1 to transparent. Is it possible to maybe begin from another color such as black?</p>