title: React Navigation switching background colors and styling StackNavigator
body: <p>I'm fairly new to React Native, but I have a simple working app with three scenes. I was previously using Navigator but it felt laggy and was excited to try out React Navigation (as in <a href="https://reactnavigation.org/" rel="noreferrer">https://reactnavigation.org/</a>). After implementing React Navigation, my background color switched from white to grey, and what was grey to white. This is a strange and shouldn't be related. However I didn't change my styles. I only implemented the new navigation and the colors changed. When I revert back to Navigator my colors return. I'm using StackNavigator. Has anyone else encountered this strange phenomenon?</p>
  
  <p>Or maybe a better question is : how do I style my header and background color in React Navigation's StackNavigator?</p>