diff --git a/source/conf.py b/source/conf.py
index 532a9c5..6533b72 100644
--- a/source/conf.py
+++ b/source/conf.py
@@ -178,4 +178,7 @@
 epub_exclude_files = ['search.html']
 
 def setup(app):
-  app.add_css_file('css/custom.css')
+    try:
+        app.add_stylesheet('css/custom.css')
+    except AttributeError:
+        app.add_css_file('css/custom.css')