[docs] Don't use Material theme in unstyled demos

[michaldudak]

Removes the @material-ui/core imports from unstyled button demos.
Discussed in #27600 (comment)

[mui-pr-bot]

No bundle size changes (experimental)

Generated by 🚫 dangerJS against b677e23

[mnajdova]

I like the changes, especially that we are moving to CSS variables. Also, the code for the demo seem isolated from any design system that we may have, which is great for the unstyled demos.

I would maybe suggest changing the title of the PR, as the changes are more than just changing the imports :)

[michaldudak]

@oliviertassinari, @mnajdova are you ok with merging this?
Perhaps @danilo-leal or @siriwatknp would have anything to add?

[danilo-leal]

A lot better! I think we also have an opportunity to display different styles leveraging the unstyled. We've already done this very nicely in the Switch customization section. This type of stuff has been very useful for me when I was in a position to consider different libraries and showcasing how different you can make it look definitely adds weight.

But just an idea for moving forward with promoting the unstyled. This is overall very good. Liked the CSS vars too!