addVars.js

module.exports = function(str) {
  return function(style) {
    style.rules = style.rules.map(function(rule) {
      if (rule.declarations) {
        rule.declarations.forEach(function(decl, i) {
          var token;
          if (decl.comment) {
            token = decl.comment.trim();
            if (token.charAt(0) === '$') {
              rule.declarations[i+1].value = token;
            }   
          }   
        }); 
      }   
      return rule;
    }); 
  }
};

button-skin.css

pure-button {
    /* $buttonBackgroundColor */
    background-color: #E6E6E6;

    /* $buttonBorderHex */
    border: 1px solid #999;  /*IE 6/7/8*/

    /* $buttonBorderRGBA */
    border: none rgba(0, 0, 0, 0);  /*IE9 + everything else*/

    /* $buttonBorderRadius */
    border-radius: 2px;

    /* $buttonColorHex */
    color: #444; /* rgba not supported (IE 8) */

    /* $buttonColorRGBA */
    color: rgba(0, 0, 0, 0.80); /* rgba supported */

    /* $buttonColorHex */
    *color: #444; /* IE 6 & 7 */

    /* $buttonPadding */
    padding: 0.5em 1.5em 0.5em;
}

replaceVars.js

var rework = require('..'),
  addVars = require('addVars.js'),
  read = require('fs').readFileSync;

var css = rework(read('button-skin.css', 'utf8'))
  .use(addVars())
  .toString()

console.log(css);

/* yields:

(note comments removed for brevity)
.pure-button {
    background-color: $buttonBackgroundColor;
    border: $buttonBorderHex;
    border: $buttonBorderRGBA;
    border-radius: $buttonBorderRadius;
    color: $buttonColorHex;
    color: $buttonColorRGBA;
    *color: $buttonColorHex;
    padding: $buttonPadding;
}

*/