A \_static file containing css templates does exist in docs\_build\\html but sphinx looks for it in docs. I copied the file to this location and the error went away.
'numpydoc' was removed from the extension list and replaced with 'sphinx.ext.napoleon'.
Traceback 5:
\begin{lstlisting}[language=Python]
WARNING: Unexpected section title.
Bug
---
\end{lstlisting}
Solution:
Napoleon (sphinx support for numpy or google docstrings) only recognises a specific subset of headings of which "Bug" is not one. Allowed headings are outlined here; \url{http://www.sphinx-doc.org/en/stable/ext/napoleon.html}. I changed the offending section to "Note" which rendered fine.