Matplotlib is a community maintained project. We welcome contributions in the form of bug reports, documentation, code, design proposals, and more.
For general information on how to contribute see https://matplotlib.org/stable/devel/.
Installing the theme itself:
After following the contribution guidelines in the Matplotlib docs, you will still need to install the theme itself to see the changes you made. At the root level of the repository run:
python -m pip install -e .
After that you will be able to build the docs locally by doing make html in the docs directory.