You can use a limited subset. cssnext isn't magic, it's purely a source transformation, there's no runtime. It can't handle the dynamic part of css variables.
Yeah, CSSNext here is basically the intersection of CSS variables and Sass variables - you can only use them in property values (like CSS), and you can only declare them at top-level (like Sass).
It's nice if you really want to utilize the current syntax, but it's less powerful than either option it's derived from.
Wow, this is awesome! I can't believe I haven't seen this. Thanks for pointing it out.