Update

components/Navbar/NavbarSubMenu.module.css

@@ -104,6 +104,20 @@
     box-shadow: 0 4px 6px -6px rgb(0 0 0 / 30%);
     opacity: 0;
     transform: translateY(-8px);
+    transition:
+      max-height 300ms ease,
+      opacity 300ms ease,
+      transform 300ms ease,
+      visibility 0s linear 300ms;
+    background-color: var(--nav-bg);
+  }
+
+  .nav-submenu.open {
+    transition:
+      max-height 300ms ease,
+      opacity 300ms ease,
+      transform 300ms ease,
+      visibility 0s linear 0s;
   }
 
   .nav-submenu-list li a:focus-visible {