[docs][joy] Error in the exemplary Codesandbox of using Material UI and Joy UI together

[hbjORbj]

• I have followed (at least) the PR section of the contributing guide.

Closes #36451

Problem

• The Codesandboxes in https://mui.com/joy-ui/guides/using-joy-ui-and-material-ui-together/ broke after we released the changes introduced in [core] Generate vars in extendTheme #35739

Codesandboxes

• Before: (1) https://codesandbox.io/s/material-ui-feat-joy-ui-vvvv59?file=/demo.tsx and (2) https://codesandbox.io/s/joy-ui-feat-material-ui-k86j2j?file=/demo.tsx
• After: (1) https://codesandbox.io/s/material-ui-feat-joy-ui-1jkvm9?file=/demo.tsx and (2) https://codesandbox.io/s/joy-ui-feat-material-ui-zhcprk?file=/demo.tsx

Preview

• https://deploy-preview-36462--material-ui.netlify.app/joy-ui/guides/using-joy-ui-and-material-ui-together/

[mui-bot]

Netlify deploy preview

• docs/data/joy/guides/using-joy-ui-and-material-ui/using-joy-ui-and-material-ui.md

Bundle size report

No bundle size changes

Generated by 🚫 dangerJS against 5a43955

[siriwatknp]

@hbjORbj I simplified the theme merging so that we don't need to use @mui/system. Also, I have updated the CodeSandbox so that we don't need to change the link.