Cover css colon selector in documentation

[sshevlyagin]

• [x] This is not a v0.x issue.
• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior 🤔

CSS colon selectors should be covered in the documentation. As per my confusion here

Current Behavior 😯

They're not covered (here)[https://material-ui.com/customization/overrides/]

I'm happy to make a pull request if y'all think this is a valid issue.

[oliviertassinari]

@sshevlyagin You don't have to use our styling solution for that. If you decide to go with JSS you can learn more about it on their documentation: https://cssinjs.org/jss-nested/.

[sshevlyagin]

Sorry if this is obvious. What would be the way to override expansionPanel firstChild and lastChild border styles using the material ui styling solution?

[oliviertassinari]

Do you want to use the theme overrides or withStyles?

[sshevlyagin]

I’m using a theme. So which one should I use?

[oliviertassinari]

@sshevlyagin You just have to replicate how it's done in the source:

    root: {
      '&:first-child': {
      },
      '&:last-child': {
      },
    }