diff --git a/agaric/templates/block/block--system-menu-block--main.html.twig b/agaric/templates/block/block--system-menu-block--main.html.twig
new file mode 100644
index 0000000..8611c9f
--- /dev/null
+++ b/agaric/templates/block/block--system-menu-block--main.html.twig
@@ -0,0 +1,2 @@
+{% set classes = classes ?? []|merge(['navbar-menu']) %}
+{% set attributes = attributes.addClass(classes) %}