make html failed with "unsupported theme option 'github_url' given" error from Sphinx. Removing the docs/_themes folder allows it to be created and populated by git clone on the first run of make html, then updated on next runs.
Fix make html
- docs/conf.py expects to create docs/_themes by cloning, otherwise
it never got populated
- Sphinx no longer fails with "unsupported theme option 'github_url'