Adrian may have a better answer, but this works for me...
If your add-on is not a Git repository, of if you would rather install the theme manually rather than as a submodule, you can just download the theme from Github and put it in your Sphinx documentation setup.
Just make sure your
conf.py file has the correct path to the theme.
So if your documentation setup looks like this:
You'd have this in your
html_theme_path = ['_themes']
html_theme = 'cidoc'
If you aren't already familiar with Sphinx I found this to be a really helpful guide: Easy and beautiful documentation with Sphinx. Along with the official doc's First Steps with Sphinx.
Thanks to Adrian for making this available!