DOC: keep color for note admonitions in sphinx theme
This doesn't change anything about the appearance in the docs, but is meant to keep the current look anticipating a change in the default color in the theme upstream (https://github.com/pandas-dev/pydata-sphinx-theme/pull/281)
@simonjayhawkins I am going to backport this for 1.2.1, not critical but just to ensure the docs keep looking the same (needed because we are using pydata-sphinx-theme master at the moment, which we should change to a pinned version once it has a new release)