title: Material UI Dark Mode primary button invisible on appbar
body: <p>I am trying to use material-ui's dark mode for a react app. I browsed through the doc and managed to activate it but i have an issue: when using a basic AppBar and a primary button on it, the primary button is "invisible" - i am guessing its the same color as the AppBar background.</p>
  
  <p>Is this normal behaviour or am I doing something wrong?</p>
  
  <p>Here's a sandbox with a quick example:</p>
  
  <p><a href="https://codesandbox.io/s/material-demo-8o3kx" rel="nofollow noreferrer">https://codesandbox.io/s/material-demo-8o3kx</a></p>
  
  <p>Thanks !</p>