def convert_epub_css_files(app: Sphinx, config: Config) -> None:
"""This converts string styled epub_css_files to tuple styled one."""
epub_css_files = [] # type: List[Tuple[str, Dict]]
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 # type: ignore
