title: How to add multiple classes in Material UI using the classes props?
body: <p>Using the <code>css-in-js</code> method to add classes to a react component, how do I add multiple components?</p>
  <p>Here is the classes variable:</p>
  <pre><code>const styles = theme =&gt; ({
    container: {
      display: 'flex',
      flexWrap: 'wrap'
    },
    spacious: {
      padding: 10
    },
  });
  </code></pre>
  <p>Here is how I used it:</p>
  <pre><code>return (&lt;div className={ this.props.classes.container }&gt;)
  </code></pre>
  <p>The above works, but is there a way to add both classes, without using the <code>classNames</code> npm package? Something like:</p>
  <pre><code>&lt;div className={ this.props.classes.container + this.props.classes.spacious}&gt;
  </code></pre>