title: Styling selected TreeItem inside a TreeView
body: <p>I'm trying to override the style of the selected TreeItem inside a Material-UI TreeView component. According to the CSS API docs, there is a <code>selected</code> selector, but when I use that, I see the entire subtree getting styled, and not just the selected item.
  What is the correct selector to use to only style the selected tree item?</p>
  <p>Code Sandbox: <a href="https://codesandbox.io/s/nostalgic-flower-e85cd?file=/src/App.js" rel="nofollow noreferrer">https://codesandbox.io/s/nostalgic-flower-e85cd?file=/src/App.js</a></p>
  <pre><code>import React from &quot;react&quot;;
  import { makeStyles } from &quot;@material-ui/core/styles&quot;;
  import TreeView from &quot;@material-ui/lab/TreeView&quot;;
  import ExpandMoreIcon from &quot;@material-ui/icons/ExpandMore&quot;;
  import ChevronRightIcon from &quot;@material-ui/icons/ChevronRight&quot;;
  import MuiTreeItem from &quot;@material-ui/lab/TreeItem&quot;;
  import { withStyles } from &quot;@material-ui/core/styles&quot;;
  
  const useStyles = makeStyles({
    root: {
      height: 240,
      flexGrow: 1,
      maxWidth: 400
    }
  });
  
  const TreeItem = withStyles({
    selected: {
      color: &quot;red&quot;
    }
  })(MuiTreeItem);
  
  export default function FileSystemNavigator() {
    const classes = useStyles();
  
    return (
      &lt;TreeView
        className={classes.root}
        defaultCollapseIcon={&lt;ExpandMoreIcon /&gt;}
        defaultExpandIcon={&lt;ChevronRightIcon /&gt;}
      &gt;
        &lt;TreeItem nodeId=&quot;1&quot; label=&quot;Applications&quot;&gt;
          &lt;TreeItem nodeId=&quot;2&quot; label=&quot;Calendar&quot; /&gt;
          &lt;TreeItem nodeId=&quot;3&quot; label=&quot;Chrome&quot; /&gt;
          &lt;TreeItem nodeId=&quot;4&quot; label=&quot;Webstorm&quot; /&gt;
        &lt;/TreeItem&gt;
        &lt;TreeItem nodeId=&quot;5&quot; label=&quot;Documents&quot;&gt;
          &lt;TreeItem nodeId=&quot;10&quot; label=&quot;OSS&quot; /&gt;
          &lt;TreeItem nodeId=&quot;6&quot; label=&quot;Material-UI&quot;&gt;
            &lt;TreeItem nodeId=&quot;7&quot; label=&quot;src&quot;&gt;
              &lt;TreeItem nodeId=&quot;8&quot; label=&quot;index.js&quot; /&gt;
              &lt;TreeItem nodeId=&quot;9&quot; label=&quot;tree-view.js&quot; /&gt;
            &lt;/TreeItem&gt;
          &lt;/TreeItem&gt;
        &lt;/TreeItem&gt;
      &lt;/TreeView&gt;
    );
  }
  </code></pre>