diff --git a/osi_model/osi_model_assets/osi_model.css b/osi_model/osi_model_assets/osi_model.css
index 8d59e91..143512a 100644
--- a/osi_model/osi_model_assets/osi_model.css
+++ b/osi_model/osi_model_assets/osi_model.css
@@ -9,6 +9,10 @@
 }
 
 summary {
-    list-style-type: none;
+    list-style-type: "+";
     cursor: pointer;
+}
+
+details[open] summary {
+    list-style-type: "-";
 }
\ No newline at end of file