Repository: create-react-app
Issue: Post CSS processing is breaking variables on css when building app
Labels: stale,needs triage,issue: bug report

<!--
    Please note that your issue will be fixed much faster if you spend about
    half an hour preparing it, including the exact reproduction steps and a demo.

    If you're in a hurry or don't feel confident, it's fine to report bugs with
    less details, but this makes it less likely they'll get fixed soon.

    In either case, please use this template and fill in as many fields below as you can.

    Note that we don't provide help for webpack questions after ejecting.
    You can find webpack docs at https://webpack.js.org/.
-->

### Describe the bug

There is some post css script changing variables like ` --white` to `#fff`

### Did you try recovering your dependencies?

Started from scratch

### Which terms did you search for in User Guide?

<!--
  There are a few common documented problems, such as watcher not detecting changes, or build failing.
  They are described in the Troubleshooting section of the User Guide:

  https://facebook.github.io/create-react-app/docs/troubleshooting

  Please scan these few sections for common problems.
  Additionally, you can search the User Guide itself for something you're having issues with:

  https://facebook.github.io/create-react-app/

  If you didn't find the solution, please share which words you searched for.
  This helps us improve documentation for future readers who might encounter the same problem.
-->
`css`, and I saw the section for post processing, but if this is intended please say so because it was working fine about 2 days ago

### Environment

<!--
  To help identify if a problem is specific to a platform, browser, or module version, information about your environment is required.
  This enables the maintainers quickly reproduce the issue and give feedback.

  Run the following command in your React app's folder in terminal.
  Note: The result is copied to your clipboard directly.

  `npx create-react-app --info`

  Paste the output of the command in the section below.
-->

```
Environment Info:

  current version of create-react-app: 4.0.3
  running from /Users/amephist/.npm/_npx/c67e74de0542c87c/node_modules/create-react-app

  System:
    OS: macOS 12.0.1
    CPU: (8) x64 Apple M1
  Binaries:
    Node: 14.18.1 - ~/.nvm/versions/node/v14.18.1/bin/node
    Yarn: Not Found
    npm: 8.1.3 - ~/.nvm/versions/node/v14.18.1/bin/npm
  Browsers:
    Chrome: 96.0.4664.55
    Edge: Not Found
    Firefox: Not Found
    Safari: 15.1
  npmPackages:
    react: ^17.0.2 => 17.0.2
    react-dom: ^17.0.2 => 17.0.2
    react-scripts: 4.0.3 => 4.0.3
  npmGlobalPackages:
    create-react-app: Not Found
```

### Steps to reproduce

<!--
  How would you describe your issue to someone who doesn’t know you or your project?
  Try to write a sequence of steps that anybody can repeat to see the issue.
-->


1. npx create-react-app
2. edit index.css add something like a `background-color: var(--white);`
3. build

### Expected behavior

<!--
  How did you expect the tool to behave?
  It’s fine if you’re not sure your understanding is correct.
  Just write down what you thought would happen.
-->

Style background-color to be `var(--white)` 

![image](https://user-images.githubusercontent.com/73475330/143612895-b5fc11b6-8982-4fa7-8156-26ef75fd7bfe.png)


### Actual behavior

Style background-color is changed to be `var(#fff)` 

![image](https://user-images.githubusercontent.com/73475330/143612847-8db61128-e1b9-47dc-87ec-5c4c2a8ffaef.png)


### Reproducible demo