Use default

docs/conf.py

@@ -101,7 +101,7 @@
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes. To override the custom theme, set this to the
 # name of a builtin theme or the name of a custom theme in html_theme_path.
-html_theme = None
+html_theme = 'default'
 
 # Custom sidebar templates, maps document names to template names.
 html_sidebars = {}