diff --git a/_doc/sphinxdoc/source/conf.py b/_doc/sphinxdoc/source/conf.py
index 9a4cc62..ab91a05 100644
--- a/_doc/sphinxdoc/source/conf.py
+++ b/_doc/sphinxdoc/source/conf.py
@@ -19,7 +19,7 @@
 # blockdiag_fontpath = '/usr/share/fonts/truetype/ipafont/ipagp.ttf'
 
 html_context = {
-    'css_files': get_default_stylesheet() + ['_static/my-styles.css'],
+    'css_files': get_default_stylesheet(['_static/my-styles.css']),
 }
 
 html_logo = "project_ico.png"