Review docs accessibility

[dcamron]

What can be better?

With #3670 we've moved away from some pydata-sphinx-theme highlighting and colors that by default were chosen to meet certain web accessibility guidelines. Some of our color combinations are now in violation of those guidelines. We should evaluate the legibility, screen reader and keyboard usability, and usability of our documentation and theme with a focus on accessibility standards. I can update this issue in the future with tools and resources for doing so.