Very interesting, and probably a useful addition to CSS.
I see, however, a problem with section 3.2, which states:
The definition of variables crosses @import boundaries. That means that the definition of a variable contained in a stylesheet B imported from a stylesheet A applies to all rules directly contained or imported into stylesheet A.
This would potentially create a problem in which variables are overwritten. Given the structure of CSS, though, and @import in particular (it must come before any style declarations), this might be acceptable.