Sphinx >7.3.2 imports themes in env, even if not set via `html_theme`

[lucyleeow]

Describe the bug

Noticed with Sphinx >7.3.2 and pydata-sphinx-theme.

1. make a clean sphinx setup with sphinx-quickstart
2. Don't change conf.py at all, default theme is 'alabaster' but pydata-sphinx-theme is installed in env
3. run make html and I get the following error:

Extension error (pydata_sphinx_theme.pygment):
Handler <function overwrite_pygments_css at 0x7ff2b211eef0> for event 'build-finished' threw an exception (exception: 'pygment_light_style')

full error message from a test failure (from sphinx-gallery/sphinx-gallery#1243):

../../../micromamba/envs/test/lib/python3.11/site-packages/sphinx/events.py:97: in emit
    results.append(listener.handler(self.app, *args))
../../../micromamba/envs/test/lib/python3.11/site-packages/pydata_sphinx_theme/pygment.py:81: in overwrite_pygments_css
    style_name = app.builder.theme.get_options()[style_key]
E   KeyError: 'pygment_light_style'

The above exception was the direct cause of the following exception:
sphinx_gallery/tests/test_gen_gallery.py:493: in test_show_memory_callable
    sphinx_app = sphinx_app_wrapper.build_sphinx_app()
sphinx_gallery/tests/conftest.py:179: in build_sphinx_app
    app.build(*args, **kwargs)
../../../micromamba/envs/test/lib/python3.11/site-packages/sphinx/application.py:353: in build
    self.events.emit('build-finished', None)
../../../micromamba/envs/test/lib/python3.11/site-packages/sphinx/events.py:108: in emit
    raise ExtensionError(
E   sphinx.errors.ExtensionError: Handler <function overwrite_pygments_css at 0x11884d6c0> for event 'build-finished' threw an exception (exception: 'pygment_light_style')

Note pygment.overwrite_pygments_css in pydata-sphinx-theme is connected to 'build-finished' (ref)

Tested and fine with Sphinx 7.3.0 and 7.3.1. If I uninstall pydata-sphinx-theme its also fine.

Could it be related to: 982681b ?

How to Reproduce

1. make a clean sphinx setup with sphinx-quickstart, include pydata-sphinx-theme in env
2. run make html

Environment Information

Platform:              linux; (Linux-5.15.0-102-generic-x86_64-with-glibc2.35)
Python version:        3.10.13 (main, Sep 11 2023, 13:44:35) [GCC 11.2.0])
Python implementation: CPython
Sphinx version:        7.3.2
Docutils version:      0.20.1
Jinja2 version:        3.1.2
Pygments version:      2.17.2

Sphinx extensions

No response

Additional context

No response

[AA-Turner]

@lucyleeow are you able to test with 3bcc669? I've changed the theme loading to be lazy via a closure rather than eager (as in 7.3.2).

A

[lucyleeow]

Fixed the problem thanks!

[AA-Turner]

Sphinx 7.3.7 has been released with fixes.

A