no theme named 'furo' found (missing theme.conf?)

[brianjmurrell]

$ /usr/bin/python3 setup.py build '--executable=/usr/bin/python3 -sP'
running build
running build_py
creating build
creating build/lib
creating build/lib/manhole
copying src/manhole/__init__.py -> build/lib/manhole
copying src/manhole/cli.py -> build/lib/manhole
running egg_info
writing src/manhole.egg-info/PKG-INFO
writing dependency_links to src/manhole.egg-info/dependency_links.txt
writing entry points to src/manhole.egg-info/entry_points.txt
writing top-level names to src/manhole.egg-info/top_level.txt
reading manifest file 'src/manhole.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.py[cod]' found anywhere in distribution
warning: no previously-included files matching '__pycache__/*' found anywhere in distribution
adding license file 'LICENSE'
adding license file 'AUTHORS.rst'
warning: no previously-included files matching '*.so' found anywhere in distribution
warning: no previously-included files matching '*.dylib' found anywhere in distribution
writing manifest file 'src/manhole.egg-info/SOURCES.txt'
copying /builddir/build/BUILD/manhole-1.8.1/src/manhole.pth -> build/lib
$ PYTHONPATH=/builddir/build/BUILD/manhole-1.8.1 sphinx-build-3 docs html
Running Sphinx v7.2.6
making output directory... done

Theme error:
no theme named 'furo' found (missing theme.conf?)

I don't really know anything about Sphinx, so I have no idea what could be going wrong here.