[Doc]: Low contrast on clicked links in dark mode

[dstansby]

Documentation Link

https://matplotlib.org/stable/

Problem

Clicked links in the dark mode theme have very low contrast with the background e.g. see "User Guide" below:

Suggested improvement

Higher contrast.

I think this is fixed in a more recent version of the pydata theme, so the fix is to remove our pin and update the dependency version (currently ~0.13.1). Doing this depends on fixing matplotlib/mpl-sphinx-theme#80