Build failures for several themes

[adamjstewart]

The following themes on https://sphinx-themes.org/ display build errors:

• Basicstrap
• Bernard
• Hachibee
• Karma
• Kotti
• Murray
• Redactor
• Sizzle
• Stanford
• Topos
• Yummy

Not sure if something changed that broke them.

[pradyunsg]

Spot checking a couple of them, they're failing with:

sphinx.errors.ThemeError: An error happened in rendering the page index.
Reason: UndefinedError("'style' is undefined")

They look like they're incompatible with Sphinx 7 and don't declare that: https://github.com/sphinx-doc/sphinx/pull/11381