Hello,
thanks for the awesome package and the great work.
I have found an issue when you have a Custom Property defined as Fallback of a Custom Property.

Given the following example:

:root {
    --my-background: #fff;
}
.three {
  background-color: var(--my-var, var(--my-background, pink));
}

Expected output should be:

:root {
    --my-background: #fff;
}
.three {
  background-color: #fff;
}

The Current output if --my-background is defined is like this:

:root {
    --my-background: #fff;
}
.three {
  background-color: var(--my-background, pink);
}

In my opinion the current output is wrong and i would like to make a PR for this if you agree.
Thanks