title: The reac-admin showNotification display the same snackbar color
body: <p>I'm using the showNotification to display messages to the user.
  The messages are shown and hidden as excpected bu the color was same (gray) with any level :</p>
  
  <pre><code>this.props.showNotification(error.message, 'error')
  
  this.props.showNotification('Done', 'success')
  </code></pre>