diff --git a/source/conf.py b/source/conf.py
index e1430c13..6028aa21 100644
--- a/source/conf.py
+++ b/source/conf.py
@@ -181,4 +181,4 @@ texinfo_documents = [
 ]
 
 def setup(app):
-    app.add_stylesheet('css/theme_extra.css')  # may also be an URL
+    app.add_css_file('css/theme_extra.css')  # may also be an URL