def stylesheet_bundle(key, files = nil)
      stylesheet_bundles[key] = StylesheetBundle.new(self, key.to_s, files)
    end
def stylesheet_bundle(key, files = nil)
      stylesheet_bundles[key] = StylesheetBundle.new(self, key.to_s, files)
    end
Set a CSS bundle
    stylesheet_bundle(:all, %w(reset grid fonts))
 @param [Symbol,String] key The bundle name
 @param [Array(String)] files The list of filenames, without extension,
   assumed to be in the public directory, under 'stylesheets'