title: Override focusVisible on MuiLink
body: <p>I would like to override the <code>focusVisible</code> property on all the (material ui) <code>Links</code> in my app.</p>
  <p>I know that I can use</p>
  <pre><code>const useStyles = makeStyles(() =&gt; ({
    focus: {
      backgroundColor: 'yellow',
    },
  }));
  </code></pre>
  <p>and then</p>
  <pre><code>const classes = useStyles();
  // ...
  &lt;Link classes={{ focusVisible: classes.focus }}&gt;{linkText}&lt;/Link&gt;
  </code></pre>
  <p>within my component, however I don't want to have to remember to do this for every <code>Link</code> in my app.</p>
  <p>I have tried using the <code>overrides</code> feature in <code>createMuiTheme</code>:</p>
  <pre><code>const myNewTheme = createMuiTheme({
    overrides: {
      MuiLink: {
        focusVisible: {
          backgroundColor: 'yellow',
        },
      },
    },
  });
  </code></pre>
  <p>but it doesn't seem to be picking this value up.</p>
  <p>What do I need to do to globally apply my chosen <code>focusVisible</code> styling to all <code>Links</code> in the app?</p>