There are various files in the repo with .old. infix which don't appear to be used

```sh
git ls-files *.old.*
docs/_static/css/custom.old.css
docs/_static/js/custom.old.js
docs/conf.old.py
```

Start with git ls-files '*.old.*' and inspect docs/_static/css/custom.old.css, docs/_static/js/custom.old.js, and docs/conf.old.py for references. Check that these files are not used by the Sphinx documentation, then remove the confirmed-unused files. Done means no obsolete .old. files remain and the documentation still builds.

Written by the indexing model from the issue text.