title: material-ui theme not applied automatically on AppBar?
body: <p>I'm trying to integrate material-ui to my project and I have some issues with custom theme settings</p>
  
  <p>I created a custom theme this way</p>
  
  <p><strong>App.js</strong></p>
  
  <pre><code>const theme = createMuiTheme({
      palette: {
          primary: green,
          secondary: red,
      },
  });
  
  class App extends Component {
  
      render() {
          return (
                  &lt;MuiThemeProvider theme={theme}&gt;
                      &lt;BrowserRouter&gt;
                          &lt;Switch&gt;
  ...}}
  </code></pre>
  
  <p>Then in a component in the substructure I create some specific css. </p>
  
  <p>Now my issue is that I'm obliged to define style <strong>appBar</strong> with a backgroud color and apply it explicitely on the AppBar component. If I don't do one of these two operations, the bg of the appBar remains light gray</p>
  
  <p>What is weired is that I get the correct green from theme.palette.primary["500"], which means the theme is correctly configured</p>
  
  <p><strong>Header.js</strong></p>
  
  <pre><code>const styles = theme =&gt; ({
      root: {
          flexGrow: 1,
      },
      grow: {
          flexGrow: 1,
      },
      appBar: {
          backgroundColor: theme.palette.primary["500"]
      },//...)}
  
  class Header extends Component {
      constructor(props) {
          super(props)
          this.classes = props.classes
      }
  
      render() {
          return (&lt;I18n&gt;
                      {(tsl, {i18n, t, ready}) =&gt; {
                          return (
                                  &lt;div className={this.classes.root}&gt;
                                      &lt;AppBar position="static" color="default" className={this.classes.appBar}&gt;
                                          &lt;Toolbar&gt;...(irrelevant code)
  </code></pre>
  
  <p>I followed the examples in <a href="https://material-ui.com/demos/app-bar/" rel="nofollow noreferrer">https://material-ui.com/demos/app-bar/</a> where the first example has the light gray color, then all the other examples have a blue bg, but there's nothing in the source code that was added to apply the blue color (in my opinion)</p>
  
  <p>Any help please? thanks folks</p>