title: How to create custom CSS classes using MUI v5 theme?
body: <p>I am using MUI v5 in my application along with <code>react-d3-tree</code> which renders SVG tree charts. It offers an API to include custom CSS class names in the <code>&lt;path&gt;</code> elements linking the tree nodes to one another. I would like to pass custom class names to <code>react-d3-tree</code> and be able to use MUI's theme variables. Thus simply importing a <code>.css</code> file will not be enough.</p>
  <p>How can I create statically named custom CSS classes using MUI v5 and take advantage of the MUI theme definition that styles other parts of my app?</p>