diff --git a/doc/source/conf.py b/doc/source/conf.py
index 28ea3264..5fe9be77 100644
--- a/doc/source/conf.py
+++ b/doc/source/conf.py
@@ -250,8 +250,9 @@ html_theme = 'sphinx_rtd_theme'
 # so a file named "default.css" will overwrite the builtin "default.css".
 html_static_path = ['_static']
 
-def setup(app):
-    app.add_stylesheet('wide.css')
+html_css_files = [
+    'wide.css',
+]
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied