- find a theme you want to copy. It will generally be a directory full of files.
- copy the directory under your project's _templates directory.
- set the html_theme in conf.py
- some docs mention setting html_theme_path as well, but it doesn't seem necessary if it's under _templates.
- There's some nice themes at https://readthedocs.org
$ ls _templates/
$ diff conf.py-orig conf.py
< html_theme = 'alabaster'
> html_theme = "sphinx_rtd_theme"
> html_theme_path = ["_themes", ] ### probably not necessary