diff --git a/docs/UsersGuide/source/conf.py b/docs/UsersGuide/source/conf.py
index b6df755443..5b5b02ea4c 100644
--- a/docs/UsersGuide/source/conf.py
+++ b/docs/UsersGuide/source/conf.py
@@ -110,7 +110,7 @@
      }
 
 def setup(app):
-    app.add_stylesheet('custom.css')  # may also be an URL
+    app.add_css_file('custom.css')  # may also be an URL
 
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.