[examples] Update examples to use CSS variables

[siriwatknp]

The CSS variables API is stable, it should be adopted in all the examples. Also, I notice some of the theming API (like using ownerState still exist) need to be cleanup.

Search keywords: examples, css variables

[wilhelmlofsten]

Hello Siriwat!

My name is Wilhelm, and I’m excited to start contributing to the MUI project!

So im new to Open source contribution, but have experience with React.js and Javascript development, and im eager to contribute to MUI!

Im trying to find a "good first issue" but have difficulty finding one, so wondering if this open issue is a good place to start?
Regarding the issue, is this then for all the examples of usage with CSS variable and updating all the examples with this API?

[github-actions]

This issue has been closed. If you have a similar problem but not exactly the same, please open a new issue.
Now, if you have additional information related to this issue or things that could help future readers, feel free to leave a comment.

[oliviertassinari]

Great, one step closer to make CSS variables the default.