Reinstate some commented-out code.

src/Avalonia.Controls/ComboBox.cs

@@ -440,15 +440,14 @@ namespace Avalonia.Controls
 
         private void SelectFocusedItem()
         {
-            throw new NotImplementedException();
-            ////foreach (ItemContainerInfo dropdownItem in ItemContainerGenerator.Containers)
-            ////{
-            ////    if (dropdownItem.ContainerControl.IsFocused)
-            ////    {
-            ////        SelectedIndex = dropdownItem.Index;
-            ////        break;
-            ////    }
-            ////}
+            foreach (var dropdownItem in GetRealizedContainers())
+            {
+                if (dropdownItem.IsFocused)
+                {
+                    SelectedIndex = IndexFromContainer(dropdownItem);
+                    break;
+                }
+            }
         }
 
         private void SelectNext()

src/Avalonia.Controls/ItemsControl.cs

@@ -419,8 +419,7 @@ namespace Avalonia.Controls
             }
             else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
             {
-                throw new NotImplementedException();
-                ////_itemContainerGenerator.ItemContainerTheme = change.GetNewValue<ControlTheme?>();
+                RefreshContainers();
             }
             else if (change.Property == ItemTemplateProperty)
             {

src/Avalonia.Controls/MenuBase.cs

@@ -95,16 +95,7 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        IEnumerable<IMenuItem> IMenuElement.SubItems
-        {
-            get
-            {
-                throw new NotImplementedException();
-                ////return ItemContainerGenerator.Containers
-                ////    .Select(x => x.ContainerControl)
-                ////    .OfType<IMenuItem>();
-            }
-        }
+        IEnumerable<IMenuItem> IMenuElement.SubItems => GetRealizedContainers().OfType<IMenuItem>();
 
         /// <summary>
         /// Gets the interaction handler for the menu.