title: Transition background color in React Navigation
body: <p>When transitioning from one Screen to another (either using card or modal mode), there's a white background that transitions its alpha from 0 o 1, during the screen to screen animation.</p>
  
  <p>I'd like to know how to change the color, if possible.</p>
  
  <p><a href="https://i.stack.imgur.com/g30n9.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/g30n9.jpg" alt="enter image description here"></a></p>
  
  <hr>
  
  <h3>Environment</h3>
  
  <ul>
  <li>React Native Navigation version: 1.0.0-beta.11</li>
  <li>React Native version: 0.45.1</li>
  <li>Platform: iOS and Android</li>
  <li>Device: iOS 10.3, iPhone 6</li>
  </ul>
  
  <hr>
  
  <h3>Some code I use to create the StackNavigation</h3>
  
  <p>Note: The modal background color was solved by @Jason Gaare's answer <a href="https://stackoverflow.com/a/45065542/976655">https://stackoverflow.com/a/45065542/976655</a>, the problem now persists on the StackNavigation</p>
  
  <pre><code>let navOptions = {
      headerMode: 'screen',
      navigationOptions: ({navigation}) =&gt; ({
          headerTintColor: '#fff',
          headerStyle: {
              backgroundColor: '#1A1A1A',
          },
          headerTitleStyle: {
              color: '#fff',
              fontFamily: 'my-font'
          },
          headerLeft: (&lt;ImageBtn
                          buttonStyle={{ .. }}
                          buttonHighlightStyle={{}}
                          source={ myImage }
                          imageStyle={{ ... }}
                          callback={navigation.goBack.bind(this, null)} /&gt;)
      })
  };
  
  const MyTab = StackNavigator({
      MyScreen1: {screen: MyScreen1},
      MyScreen2: {screen: MyScreen2},
      MyScreen3: {screen: MyScreen3},
      MyScreen4: {screen: MyScreen4},
  }, navOptions);
  </code></pre>