expand_stylesheet_sources

# File actionpack/lib/action_view/helpers/asset_tag_helper.rb, line 648
        def expand_stylesheet_sources(sources, recursive)
          if sources.first == :all
            collect_asset_files(STYLESHEETS_DIR, ('**' if recursive), '*.css')
          else
            sources.collect do |source|
              determine_source(source, @@stylesheet_expansions)
            end.flatten
          end
        end