docs: def add_css(self, subdir, file_name_prefix):
          """Add a css file for this resource.
  
          If self.minify is True, checks if the .min.css file exists. If not, falls back to non-minified file. If that
          file also doesn't exist, IOError is raised.
  
          Positional arguments:
          subdir -- sub directory name under the resource's main directory (e.g. css or js, or an empty string).
          file_name_prefix -- file name without the file extension.
          """
          suffix_maxify = '.css'
          suffix_minify = '.min.css'
          if self.minify and self.file_exists(subdir, file_name_prefix, suffix_minify):
              self.resources_css.append(posixpath.join(self.DIR, subdir, file_name_prefix + suffix_minify))
          elif self.file_exists(subdir, file_name_prefix, suffix_maxify):
              self.resources_css.append(posixpath.join(self.DIR, subdir, file_name_prefix + suffix_maxify))
          else:
              file_path = os.path.join(self.STATIC_DIR, self.DIR, subdir, file_name_prefix + suffix_maxify)
              raise IOError('Resource file not found: {0}'.format(file_path))