Resolve :root variables when combined with class definitions

[blvdmitry]

I've seen a couple of issues about resolving variables attached to specific classes and that's true indeed that it's impossible to resolve nesting cases in build time. My use case is a bit different from this, but similar topic:

:root, .foo {
  --color-red: red;
}

Couldn't make this definition work in the build time even for :root, while it should be possible to resolve at least that part in the build time.