Raise `ThemeError` for invalid theme sections

CHANGES.rst

@@ -41,6 +41,8 @@ Incompatible changes
   Patch by Adam Turner.
 * #12593: Raise an error for invalid :confval:`html_sidebars` values.
   Patch by Adam Turner.
+* #12593: Raise an error in :py:func:`!Theme.get_config` for invalid sections.
+  Patch by Adam Turner.
 
 Deprecated
 ----------

sphinx/theming.py

@@ -121,17 +121,11 @@ class Theme:
         elif section == 'options':
             value = self._options.get(name, default)
         else:
-            # 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)'
+                'are not supported (tried to get a value from %r).'
             )
-            logger.info(msg, section)
-            value = default
+            raise ThemeError(msg)
         if value is _NO_DEFAULT:
             msg = __('setting %s.%s occurs in none of the searched theme configs') % (
                 section,

tests/test_theming/test_theming.py

@@ -72,6 +72,9 @@ def test_theme_api(app, status, warning):
     assert theme.get_config('theme', 'foobar', 'def') == 'def'
     with pytest.raises(ThemeError):
         theme.get_config('theme', 'foobar')
+    # nonexisting section
+    with pytest.raises(ThemeError):
+        theme.get_config('foobar', 'foobar')
 
     # options API