sphinx_rtd_theme is no longer a hard dependency since version 1.4.0

I found another issue, I had two installations of sphinx, one via pip and another one via brew. Deleting one (I chose to remove installation done via brew ) solved this issue.

brew remove sphinx-doc

If it still doesnt work, try uninstalling and reinstalling

pip3  uninstall sphinx sphinx_rtd_theme
pip3  install sphinx sphinx_rtd_theme

On my Mac OS X installation, I encountered this problem, despite following the simple instructions to the letter.

Others have reported having the problems as well, on this Github issue: https://github.com/readthedocs/sphinx_rtd_theme/issues/618

In my case, the problem turned out to be an issue with my python environment (I was using the default system python 2.7 for Mac). I ended up switching to pyenv with python 3.7.3, and that fixed the issue.

Instructions for Mac (from https://opensource.com/article/19/5/python-3-default-mac):

$ brew install pyenv 

$ pyenv install 3.7.3

$ pyenv global 3.7.3

$ pyenv version
3.7.3 (set by /Users/mbbroberg/.pyenv/version)

$ echo -e 'if command -v pyenv 1>/dev/null 2>&1; then\n  eval "$(pyenv init -)"\nfi' >> ~/.zshrc

To verify that it worked:

$ which python
/Users/mbbroberg/.pyenv/shims/python

$ python -V
Python 3.7.3

$ pip -V
pip 19.0.3 from /Users/mbbroberg/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip (python 3.7)

Don't forget to pip install sphinx sphinx_rtd_theme afterward.

In my case (building from RTD theme source), the problem seemed to be caused by a change in behaviour of the html_theme_path config value in conf.py. Whilst paths relative to the makefile directory used to be accepted, paths must now be relative to the configuration directory:

html_theme_path

A list of paths that contain custom themes, either as subdirectories or as zip files. Relative paths are taken as relative to the configuration directory.

New in version 0.6.

— Sphinx documentation > Configuration

I had the same problem on a Windows 10 Pro laptop. I had installed Python 3.9 from the Windows store. I had to first find where Python and pip applications were installed and add that to the PATH variable. This was in a folder that was looked:

c:\users\<username>\appdata\local\programs\python\python39\Scripts

Then I installed sphinx using:

pip install -U sphinx

Then I had to set the SPHINXBUILD env variable to the right value:

SPHINXBUILD="c:\users\<username>\appdata\local\programs\python\python39\Scripts\sphinx-build.exe"

When I tried to use the sphinx_rtd_theme I kept getting the error:

Theme error:
sphinx_rtd_theme is no longer a hard dependency since version 1.4.0. Please install it 
manually.(pip install sphinx_rtd_theme)

I ran the following command:

pip install sphinx_rtd_theme

But when I tried to run make html it gave the error:

Running Sphinx v3.5.1

Configuration error:
There is a syntax error in your configuration file: bad input (conf.py, line 57)

Finally I had to make the following changes in my conf.py file:

# Add this to the top of the conf.py file
import os
import sys
sys.path.insert(0, os.path.abspath('C:\\Users\\ <username>\\AppData\\Local\\Packages\\PythonSoftwareFoundation.Python.3.9_qbz5n2kfra8p0\\LocalCache\\local-packages\\Python39\\site-packages'))
import sphinx_rtd_theme

extensions = [
   "sphinx_rtd_theme",
]

It looks like sphinx-build is not able to get the location of the sphinx_rtd_theme package unless you give the explicit path in the manner above. “¯_(ツ)_/¯“