Return the default value for unsupported theme configuration sections

CHANGES.rst

@@ -7,7 +7,9 @@ Bugs fixed
 * #12299: Defer loading themes defined via entry points until
   their explicit use by the user or a child theme.
   Patch by Adam Turner.
-
+* #12305: Return the default value for ``theme.get_config()`` with
+  an unsupported theme configuration section.
+  Patch by Adam Turner.
 
 Release 7.3.6 (released Apr 17, 2024)
 =====================================

sphinx/theming.py

@@ -121,7 +121,17 @@ def get_config(self, section: str, name: str, default: Any = _NO_DEFAULT) -> Any
         elif section == 'options':
             value = self._options.get(name, default)
         else:
-            value = _NO_DEFAULT
+            # https://github.com/sphinx-doc/sphinx/issues/12305
+            # For backwards compatibility when attempting to read a value
+            # from an unsupported configuration section.
+            # xref: RemovedInSphinx80Warning
+            msg = __(
+                'Theme configuration sections other than [theme] and [options] '
+                'are not supported, returning the default value instead '
+                '(tried to get a value from %r)'
+            )
+            logger.info(msg % section)
+            value = default
         if value is _NO_DEFAULT:
             msg = __('setting %s.%s occurs in none of the searched theme configs') % (
                 section,