diff '--color=auto' -ru a/doc/conf.py b/doc/conf.py
--- a/doc/conf.py	2021-07-24 17:12:08.891743446 -0400
+++ b/doc/conf.py	2021-07-24 17:12:38.231700640 -0400
@@ -27,7 +27,7 @@
 
 # https://docs.readthedocs.io/en/latest/guides/adding-custom-css.html
 def setup(app):
-    app.add_stylesheet('css/custom.css')
+    app.add_css_file('css/custom.css')
     #app.add_javascript('js/custom.js')