use :not(:has(...)) to resolve subtree specificity issues when using CSS vars

[rossipedia]

I believe we can leverage :not(:has(...)) to resolve the specificity issues outlined in #43824, as the selector should only match the parent that does not have a child element with a conflicting theme selector applied to it.

Resolves #43824

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

Exampe

• https://github.com/rossipedia/mui-css-vars-with-patch
• https://stackblitz.com/~/github.com/rossipedia/mui-css-vars-with-patch

[rossipedia]

After more investigation, this will not work as expected, and will remove theme.applyStyles(...) from applying based on the root theme selector.