diff --git a/sphinxcontrib/versioning/sphinx_.py b/sphinxcontrib/versioning/sphinx_.py
index a90587d4f..291884b67 100644
--- a/sphinxcontrib/versioning/sphinx_.py
+++ b/sphinxcontrib/versioning/sphinx_.py
@@ -57,7 +57,10 @@ def builder_inited(app):
 
         # Add versions.html to sidebar.
         if '**' not in app.config.html_sidebars:
-            app.config.html_sidebars['**'] = StandaloneHTMLBuilder.default_sidebars + ['versions.html']
+            if 'html_sidebars' in StandaloneHTMLBuilder(app).get_theme_config()[1].keys():
+                app.config.html_sidebars['**'] = StandaloneHTMLBuilder(app).get_theme_config()[1]['html_sidebars'] + ['versions.html']
+            else:
+                app.config.html_sidebars['**'] = ['versions.html']
         elif 'versions.html' not in app.config.html_sidebars['**']:
             app.config.html_sidebars['**'].append('versions.html')