[css mode] teach it about CSS variables

mode/css/css.js

@@ -57,6 +57,11 @@ CodeMirror.defineMode("css", function(config, parserConfig) {
       if (/[\d.]/.test(stream.peek())) {
         stream.eatWhile(/[\w.%]/);
         return ret("number", "unit");
+      } else if (stream.match(/^-[\w\\\-]+/)) {
+        stream.eatWhile(/[\w\\\-]/);
+        if (stream.match(/^\s*:/, false))
+          return ret("variable-2", "variable-definition");
+        return ret("variable-2", "variable");
       } else if (stream.match(/^\w+-/)) {
         return ret("meta", "meta");
       }