Use add_css_file instead of add_stylesheet as requested by Sphinx

docs/conf.py

@@ -63,4 +63,4 @@ html_static_path = ['_static']
 
 
 def setup(app):
-    app.add_stylesheet('css/custom.css')  # may also be an URL
+    app.add_css_file('css/custom.css')  # may also be an URL