add css

app/assets/stylesheets/application.css

@@ -12,4 +12,5 @@
  *
  *= require_tree .
  *= require_self
+ *= stub “active_admin"
  */

app/javascript/packs/active_admin.js

@@ -3,3 +3,4 @@
 import "../stylesheets/active_admin";
 
 import "@activeadmin/activeadmin";
+import "active_material"

app/javascript/packs/active_admin.scss

@@ -0,0 +1 @@
+@import "active_material/src/stylesheets/active_material.scss";