title: Material UI migration to V5: how to access theme properties types in @mui/material v5
body: <p>After migrating from @material-ui version 4 to 5 I'm not accessing the theme properties types, does anyone know what I need to do to solve this?</p>
  <p><strong>Material-ui v4:</strong> I can access the theme properties!
  <a href="https://i.stack.imgur.com/zShZm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/zShZm.png" alt="enter image description here" /></a></p>
  <p><a href="https://codesandbox.io/s/material-ui-core-v4-kb2zuq?file=/src/App.js" rel="nofollow noreferrer">See in CodeSandbox: Material-ui v4</a></p>
  <p><strong>Material v5:</strong> I can't access theme properties!
  <a href="https://i.stack.imgur.com/R8azh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R8azh.png" alt="enter image description here" /></a></p>
  <p><a href="https://codesandbox.io/s/mui-material-v5-tsfxjk?file=/src/App.js" rel="nofollow noreferrer">See in CodeSandbox: Material-ui v5</a></p>
  <p>In both examples the theme is working, only the properties are not showing in version 5.</p>
  <p>Thanks for your help!</p>