code: def compress_css(self, paths, output_filename, variant=None, **kwargs):
          
          css = self.concatenate_and_rewrite(paths, output_filename, variant)
          compressor = self.css_compressor
          if compressor:
              css = getattr(compressor(verbose=self.verbose), 'compress_css')(css)
          if not variant:
              return css
          elif variant == "datauri":
              return self.with_data_uri(css)
          else:
              raise CompressorError("\"%s\" is not a valid variant" % variant)
docs: Concatenate and compress CSS files