[Bug]: Variable in one css file used in another one removed

[fadomire]

Describe the bug

I have 2 css files that i pass to purgeCSS

1. define CSS variables
   :root { --used-in-other-css-file: red; }
2. define class that use those variables
   .button { color: var(--used-in-other-css-file) }

Depending on the order in the array of files i pass to purgeCss either my variable is removed or not.

If i pass variable file then class file, then variable is removed.
If i pass class file then variable file, my variable is preserved.

To Reproduce

Here is a git patch on this repo to reproduce the issue via tests
Changes-On-b8bb2a.patch

Expected Behavior

my variable is always preserved

Environment

6.0.0 purgecss
nodejs env

Code of Conduct

• I agree to follow this project's Code of Conduct

[jeffijoe]

This is also the case when the variable is referenced in content, for example, a JavaScript file might read a variable's value.