title: How does one use or get started with ...theme.mixins.toolbar in Material-ui?
body: <p>I cannot find any documentation for ...theme.mixins.toolbar in the material-ui docs. What does this do? How do I change or use it?</p>
  
  <p><a href="https://i.stack.imgur.com/KnuAv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/KnuAv.png" alt="enter image description here"></a></p>