Close #9683: Revert the removal of `add_stylesheet()` API

It will be kept until the Sphinx-6.0 release.

Note: Now it emits a warning instead of DeprecationWarning to let the
users know the deprecation.

CHANGES

@@ -22,6 +22,8 @@ Features added
 * #9691: C, added new info-field ``retval``
   for :rst:dir:`c:function` and :rst:dir:`c:macro`.
 * C++, added new info-field ``retval`` for :rst:dir:`cpp:function`.
+* #9683: Revert the removal of ``add_stylesheet()`` API.  It will be kept until
+  the Sphinx-6.0 release
 
 Bugs fixed
 ----------

doc/extdev/deprecated.rst

@@ -1207,7 +1207,7 @@ The following is a list of deprecated interfaces.
 
    * - :meth:`~sphinx.application.Sphinx.add_stylesheet()`
      - 1.8
-     - 4.0
+     - 6.0
      - :meth:`~sphinx.application.Sphinx.add_css_file()`
 
    * - :meth:`~sphinx.application.Sphinx.add_javascript()`

sphinx/application.py

@@ -1046,6 +1046,26 @@ class Sphinx:
         if hasattr(self.builder, 'add_css_file'):
             self.builder.add_css_file(filename, priority=priority, **kwargs)  # type: ignore
 
+    def add_stylesheet(self, filename: str, alternate: bool = False, title: str = None
+                       ) -> None:
+        """An alias of :meth:`add_css_file`.
+
+        .. deprecated:: 1.8
+        """
+        logger.warning('The app.add_stylesheet() is deprecated. '
+                       'Please use app.add_css_file() instead.')
+
+        attributes = {}  # type: Dict[str, Any]
+        if alternate:
+            attributes['rel'] = 'alternate stylesheet'
+        else:
+            attributes['rel'] = 'stylesheet'
+
+        if title:
+            attributes['title'] = title
+
+        self.add_css_file(filename, **attributes)
+
     def add_latex_package(self, packagename: str, options: str = None,
                           after_hyperref: bool = False) -> None:
         r"""Register a package to include in the LaTeX source code.