DOC Sphinx global var `style` removed in Sphinx 7.0.0

### Describe the issue linked to the documentation

Following the [doc contribution guide](https://scikit-learn.org/dev/developers/contributing.html#building-the-documentation) to build docs, the latest Sphinx version (7.0.1) is installed but upon build the following error occurs:

<details>

<summary>Error message</summary>

```
Traceback (most recent call last):
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/html/__init__.py", line 1093, in handle_page
    output = self.templates.render(templatename, ctx)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/jinja2glue.py", line 196, in render
    return self.environment.get_template(template).render(context)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/jinja2/environment.py", line 1301, in render
    self.environment.handle_exception()
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
    {%- extends "layout.html" %}
  File "/home/<usr>/Documents/scikit-learn/doc/themes/scikit-learn-modern/layout.html", line 36, in top-level template code
    <link rel="stylesheet" href="{{ pathto('_static/' + style, 1) }}" type="text/css" />
jinja2.exceptions.UndefinedError: 'style' is undefined

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/cmd/build.py", line 285, in build_main
    app.build(args.force_all, args.filenames)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/application.py", line 351, in build
    self.builder.build_update()
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 294, in build_update
    self.build(to_build,
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 361, in build
    self.write(docnames, list(updated_docnames), method)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 566, in write
    self._write_parallel(sorted(docnames),
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 593, in _write_parallel
    self.write_doc(firstname, doctree)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/html/__init__.py", line 667, in write_doc
    self.handle_page(docname, ctx, event_arg=doctree)
  File "/home/<usr>/miniconda3/envs/sklearn-env/lib/python3.9/site-packages/sphinx/builders/html/__init__.py", line 1100, in handle_page
    raise ThemeError(__("An error happened in rendering the page %s.\nReason: %r") %
sphinx.errors.ThemeError: An error happened in rendering the page about.
Reason: UndefinedError("'style' is undefined")
```
</details>

It seems read the docs has the same error: https://github.com/readthedocs/readthedocs.org/issues/10279

There is a PR to fix: https://github.com/readthedocs/sphinx_rtd_theme/pull/1464

Can't quite tell how this fixes the problem, note the stylesheet section of their 'layout.html' is guarded by version conditional, see: https://github.com/readthedocs/sphinx_rtd_theme/pull/1464#issuecomment-1563162074

### Suggest a potential alternative/fix

:thinking:

COMMENT:
Note `styles` was introduced in 5.1, after the min dependency 4.0.1.
Will leave this issue for reference (as we may want to update our Sphinx version in future), though currently this does not need to be fixed since we have pinned Sphinx to 6.0.0 due to https://github.com/scikit-learn/scikit-learn/pull/25504

Labels: Bug, Documentation