The current .png is presumably the wrong shape and looks flattened. Plus, mkdocs complained that 'site_favicon' was an unrecognised configuration name, so I used the method described in the docs instead: https://www.mkdocs.org/user-guide/customizing-your-theme/#using-the-theme_dir
As documented on how to override template blocks: https://www.mkdocs.org/user-guide/customizing-your-theme/#overriding-template-blocks This is a big diff. I tested it by generating docs before and after the change, and they were functionally identical.