title: Material-UI: "The key provided to the classes property is not implemented"
body: <p>I am using the withStyles() HOC to override some MUI component styles, theme and breakpoints.</p>
  
  <p>There is obviously something I do not understand here as I keep getting errors such as this one:</p>
  
  <blockquote>
    <p>Warning: Material-UI: the key <code>tab</code> provided to the classes property
    is not implemented in Items.</p>
    
    <p>You can only override one of the following:
    card,details,content,cover,avatar,lock</p>
  </blockquote>
  
  <p>Example code: <a href="https://codesandbox.io/s/6xwz50kxn3" rel="noreferrer">https://codesandbox.io/s/6xwz50kxn3</a></p>
  
  <p>I have a <code>&lt;List /&gt;</code> component and its child <code>&lt;Items /&gt;</code>.</p>
  
  <p>My intention is to apply the styles in the demo.js file only to the <code>&lt;List /&gt;</code> component, and the styles in the demoChild.js to the <code>&lt;Items /&gt;</code> Component.</p>
  
  <p>I would really appreciate an explanation of what I'm doing wrong, and maybe a solution?</p>
  
  <p>Note: I have found <a href="https://stackoverflow.com/questions/45284597/overriding-with-classes-in-material-ui-v1-0-0-beta-1-shows-the-key-provided-to">other posts</a> with the same error, but they seem to have something different to my example.</p>