NLTK Theme is a Sphinx theme forked from the wonderful Insegel, designed for the NLTK documentation. Feel free to have a look at the theme documentation or look at the theme in action over at www.nltk.org.
This theme is not necessarily intended for reuse. I would advise looking at Insegel if you'd like to use a similar theme.
That said, you may install it via pip:
pip install nltk_theme
Then, you can tell Sphinx to use the theme by adding:
html_theme = 'nltk_theme'
to your project's conf.py.