Raise a useful error if `theme.conf` doesn't exist (#11671)

tests/test_theming.py

@@ -6,7 +6,7 @@ import alabaster
 import pytest
 
 import sphinx.builders.html
-from sphinx.theming import ThemeError
+from sphinx.theming import Theme, ThemeError
 
 
 @pytest.mark.sphinx(
@@ -62,6 +62,13 @@ def test_theme_api(app, status, warning):
     assert not os.path.exists(themedir)
 
 
+def test_nonexistent_theme_conf(tmp_path):
+    # Check that error occurs with a non-existent theme.conf
+    # (https://github.com/sphinx-doc/sphinx/issues/11668)
+    with pytest.raises(ThemeError):
+        Theme('dummy', str(tmp_path), None)
+
+
 @pytest.mark.sphinx(testroot='double-inheriting-theme')
 def test_double_inheriting_theme(app, status, warning):
     assert app.builder.theme.name == 'base_theme2'