[docs] Fix branding theme leaking on the templates

[mnajdova]

This PR adds the MD theme on the templates so that the docs theme does not leak its styles on the templates.

[mui-pr-bot]

No bundle size changes (experimental)

Generated by 🚫 dangerJS against af0a14e

[oliviertassinari]

The alternative approach would have been to remove the custom them from the _app.js and to only apply it with the pages' top-level component. For instance, https://deploy-preview-28120--material-ui.netlify.app/performance/table-mui/ is not using the default theme, or say, if a maintainer would like to copy and paste a reproduction in an empty Next.js's page to work on it, he won't see the default theme either.

Oh actually, it might be what #28119 is doing, scoping the global theme provider away from _app.js

[oliviertassinari]

I'm hitting this pain again during the Lisbon company Hackathon. Our code behaves differently in a raw Next.js docs page compared to codesandbox. @mnajdova could have the _app.js barer? So that by default, the default theme is used?

[mnajdova]

I'm hitting this pain again during the Hackathon. Our code behaves differently in a raw Next.js docs page compared to codesandbox. @mnajdova could have the _app.js barer? So that by default, the default theme is used?

Putting it on my TODO list.