Describe the bug

In my concrete case, the CSS file is added with add_css_file()
in my nbsphinx extension: https://github.com/spatialaudio/nbsphinx/blob/53be5e632f0c9483ba65ba6690da9b137f9c1120/src/nbsphinx/__init__.py#L1655

How to Reproduce

Check out https://github.com/spatialaudio/nbsphinx/ and add this to doc/conf.py:

html_static_path = ['my-static-path']

Create a file my-static-path/nbsphinx-gallery.css with some CSS overrides, like e.g. in https://github.com/Substra/substra-documentation/blob/d2785aa82d46686d82ab66d2c24f95af86d28ce6/docs/source/static/nbsphinx-gallery.css.

Run Sphinx.

Look at one of the thumbnail galleries.

Environment Information

Any Sphinx version after ae206694e68bea074aca633ea0d32e9ed882a95f, appearing first in release 7.1.0.

Sphinx extensions

`nbsphinx`, but this should happen with any extension that adds some CSS.

Additional context

This was originally reported here: spatialaudio/nbsphinx#778

I have bisected it, and the culprit is ae20669, which is part of #11415.