code: def convert_epub_css_files(app: Sphinx, config: Config) -> None:
      """Convert string styled epub_css_files to tuple styled one."""
      epub_css_files: list[tuple[str, dict[str, Any]]] = []
      for entry in config.epub_css_files:
          if isinstance(entry, str):
              epub_css_files.append((entry, {}))
          else:
              try:
                  filename, attrs = entry
                  epub_css_files.append((filename, attrs))
              except Exception:
                  logger.warning(
                      __("invalid css_file: %r, ignored"), entry
                  )
                  continue
  
      config.epub_css_files = epub_css_files
  
imports: from __future__ import annotations
  import html
  import os
  import re
  import time
  from os import path
  from typing import TYPE_CHECKING, Any, NamedTuple
  from sphinx import package_dir
  from sphinx.builders import _epub_base
  from sphinx.config import ENUM, Config
  from sphinx.locale import __
  from sphinx.util import logging
  from sphinx.util.fileutil import copy_asset_file
  from sphinx.util.osutil import make_filename
label_desc: Use image node_id 2 for calling a global function with example usage: convert_epub_css_files(app, config) without return types