gulpfiles.js

/* gulp stylesheets or gulp stylesheets --production */
gulp.task("stylesheets", function() {
  return gulp.src("./assets/css/admin.css")
  .pipe(options.production ? plumber() : gutil.noop())
  .pipe(cssnext({
    sourcemap : !options.production,
    browsers : ['ff >= 20', 'ie >= 8', 'safari >= 5.1', 'opera >= 12', 'chrome >=20'],
    features: {
      customProperties: true,
      calc: true ,
      customMedia: true,
      mediaQueriesRange: true,
      customSelectors: true,
      colorFunction:true,
      colorHexAlpha: true,
      colorHwb:true,
      colorRebeccapurple: true,
      fontVariant: true,
      filter: true,
      rem: true
    }
  }))
  .pipe(options.production ? minifyCSS({keepSpecialComments:0}) : gutil.noop())
  .pipe(gulp.dest("./assets/build/stylesheets"))
});