code: def test_epub_css_files(app):
      app.build(force_all=True)
  
      # epub_css_files
      content = (app.outdir / "index.xhtml").read_text(encoding="utf8")
      assert (
          '<link rel="stylesheet" type="text/css" href="_static/css/epub.css" />'
          in content
      )
  
      # files in html_css_files are not outputted
      assert (
          '<link rel="stylesheet" type="text/css" href="_static/css/style.css" />'
          not in content
      )
      assert (
          '<link media="print" rel="stylesheet" title="title" type="text/css" '
          'href="https://example.com/custom.css" />' not in content
      )
  
imports: import os
  import subprocess
  from pathlib import Path
  from subprocess import CalledProcessError
  from xml.etree import ElementTree
  import pytest
  from sphinx.builders.epub3 import _XML_NAME_PATTERN
label_desc: Use image node_id 9 for calling a global function with example usage: test_epub_css_files(app) without return types