add support for global CSS variables

[robkorv]

https://developer.mozilla.org/en-US/docs/Web/CSS/:root and https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties

I think

:root {
  --text-color: 16px;
}

Should be converted to:

#pefix {
  --text-color: 16px;
}