title: how theme.spacing() is functioning?
body: <p>As per the document in material-ui,it is providing space between the components.  But I could get the way it is getting height and width here:</p>
  
  <pre><code>width: theme.spacing(20),
  height: theme.spacing(15)
  </code></pre>
  
  <p>How is <code>theme.spacing()</code> setting the width and height here?</p>