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:
Potentially we should add a note to our documentation guide, as anyone following the suggested `pip install sphinx` would get the latest version and would fail to build the docs?

Labels: Bug, Documentation