> Shouldn’t the html target depend on venv then?

Back when we added support for blurb, I tried to make it such that "make html" could easily use already installed versions of sphinx-build and blurb and not require a venv.  I had a particular use case for that.  But if someone wants to modify the Docs Makefile to require using the venv recipe, I wouldn't object too strongly.  Another approach, I guess, would be to check that the python-docs-theme is available to sphinx-build and give a better message: not sure how best to do that in the current Makefile.