[core] Enable stylelint on CSS files

[oliviertassinari]

I noticed we didn't lint the .css files after my IDE reported issues opening them (using https://marketplace.visualstudio.com/items?itemName=stylelint.vscode-stylelint). It seems that we should start to lint those. There are too many rules to fix for me to make sense to do it, but I went with the simplest, quick win path:

• lint those path
• test if it works by fixing a few errors
• use the current number of warnings as the current limit accepted, so the problem doesn't grow

I think next we could:

• iterate on the rule to see the ones that make sense
• fix them all
• remove the warning default severity and max error count
• do the same on https://github.com/mui/material-ui

[oliviertassinari]

Duplicated value, seems wrong

[mui-bot]

Netlify deploy preview

https://deploy-preview-1184--base-ui.netlify.app/

Generated by 🚫 dangerJS against c686b2c

[michaldudak]

I'd disable the following rules:

• declaration-empty-line-before, custom-property-empty-line-before - makes long rulesets more readable
• import-notation - the bundler understands the simple notation we use

And autofix all the remaining violations.

[oliviertassinari]

I'd disable the following rules:

As a follow up PR though, no? First goal here is to avoid the count to increase. To note that the first one can come from https://github.com/mui/material-ui/pull/44874/files#diff-2633572b1eb62578838877abeb39ff8053af5fa47a1a731129b889db37e59610R15 if we merge this. And the second one, we could add it to the monorepo config as I guess it's relevant for everyone?

[michaldudak]

Yeah, could be a follow-up, I don't mind.