replaceCss(archive, resolver) {
    var replaced = [];
    archive = archive || this.settings.archive;
    resolver = resolver || this.settings.resolver;
    this.cssUrls.forEach(function (href) {
      var replacement = this.createCssFile(href, archive, resolver).then(function (replacementUrl) {
        // switch the url in the replacementUrls
        var indexInUrls = this.urls.indexOf(href);

        if (indexInUrls > -1) {
          this.replacementUrls[indexInUrls] = replacementUrl;
        }
      }.bind(this));
      replaced.push(replacement);
    }.bind(this));
    return Promise.all(replaced);
  }
Replace URLs in CSS resources
@private
@param  {Archive} [archive]
@param  {method} [resolver]
@return {Promise}