title: make a custom class globally useable for material-ui
body: <p>I found no single example on how to reuse classes using <code>makeStyles()</code> in material-ui.</p>
  <p>Say I have a class called <code>resizable</code> or <code>floating</code></p>
  <p>I have to define <code>makeStyles()</code> inside every component which shares a specific class and duplicate the code.</p>
  <p>The theming function does not provide any option to do this.
  All the theming examples show how to use the existing api props.</p>
  <p>Maybe a context provider should be used to share styles?
  But wasn't the point of <code>useTheme()</code> exactly this? I don't understand why.</p>
  <p>Anyone experienced with material-ui?</p>