Note that this is in the styles used for local and ReadTheDocs PR preview builds—not the styles used on docs.djangoproject.com.
If there’s a quick fix in the “old official theme” that’s included with Django’s source, that’s great. But if it starts getting complicated, it might be more productive to try to pull in the “new” styles from djangoproject.com instead. See related discussion in Why does local django docs have a different styling/theme than the official one? and ticket #37138 (Outdated docs CSS in local and preview builds) – Django.