[docs] Hotfix missing styles in dark mode

[siriwatknp]

Preview: https://deploy-preview-35179--material-ui.netlify.app/material-ui/api/alert/

Reported in #34976 (comment).

@siriwatknp I have found a regression from this PR (looking a bit at https://mui-org.slack.com/archives/C042VP80PT5/p1667372692296329):

The commit on master just before this PR: https://636d3dbaea7a9900083c33e1--material-ui.netlify.app/material-ui/api/alert/

This PR: https://deploy-preview-34976--material-ui.netlify.app/material-ui/api/alert/

After:

My mistake. Forgot the case where the tokens have different values between color modes.

will do a hotfix once the CIs are green.

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[cherniavskii]

Noticed this in MUI X docs as well: https://deploy-preview-6773--material-ui-x.netlify.app/x/api/data-grid/data-grid/
Thanks for the fix, will upgrade the monorepo on our side! 🎉

[oliviertassinari]

@siriwatknp To confirm if I understand this correctly: Once we have the CssVarsProvider on all the pages, we can remove the whole ':where(.mode-dark) &': { block?

[siriwatknp]

@siriwatknp To confirm if I understand this correctly: Once we have the CssVarsProvider on all the pages, we can remove the whole ':where(.mode-dark) &': { block?

Not the whole block (otherwise the dark mode does not work), we could replace the .mode-dark with the data attribute from the theme.

[oliviertassinari]

@siriwatknp Right, my bad. So more like this?

Once we have the CssVarsProvider provider on all the pages, we can then remove all the CSS properties in ':where(.mode-dark) &': { that rely on a var(--muidocs-xxx). So effectively we will be able to remove 80% of the code inside this block.

It sounds good :).

Ideally, I think that it would be great to be able to replace the var(--muidocs-xxx) with theme.vars.xxx values for type safety. I imagine that it would allow us to catch future regressions.