title: how to style react-native-router-flux?
body: <p>I am using react-native-router-flux 4.0.0-beta.17 for my learning project. I need to customize the header. for instance the background color, the title alignment, etc. I couldn't find a good document about it. One of them had something like </p>
  
  <pre><code> &lt;Router sceneStyle={{backgroundColor: '#81b71a'}}&gt;
       &lt;Scene key="root"&gt;
          &lt;Scene key='login' component={LoginForm} title='Please Login :)' /&gt;
       &lt;/Scene&gt;
   &lt;/Router&gt;
  </code></pre>
  
  <p>but it doesn't do anything.</p>
  
  <p>Please give me some reference about good docs and also if possible, some information about how to style the router.  where can I find a comprehensive document?</p>