You can use PostCSS + CSSNext + Import plugin, which replaces preprocessors right now. Without that I wouldn't recommend to use CSS variables because of browser support, so you should transpile your new CSS to the old CSS3.
I am already using it. There are no much difference. Your code just now is a plain CSS and later you will be able to use it without changing anything. It is also much faster then using python or something to build sass.