Cannot make html

[kogorman]

On a recent Xubuntu, I am unable to "make html". I have just installed python3-sphinx from the repositories, and I'm using the latest master of pysgf from GitHub. It fails with

kevin@brandyn:~/Downloads/pysgf-master/docs$ make html
Running Sphinx v4.3.2
loading translations [en]... done
making output directory... done
WARNING: sphinx_rtd_theme (< 0.3.0) found. It will not be available since Sphinx-6.0

Theme error:
no theme named 'sphinx_rtd_theme' found (missing theme.conf?)
make: *** [Makefile:20: html] Error 2
kevin@brandyn:~/Downloads/pysgf-master/docs$