Preserve container themes set locally.

src/Avalonia.Controls/ItemsControl.cs

@@ -67,6 +67,9 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<IBinding?> DisplayMemberBindingProperty =
             AvaloniaProperty.Register<ItemsControl, IBinding?>(nameof(DisplayMemberBinding));
 
+        private static readonly AttachedProperty<ControlTheme?> AppliedItemContainerTheme =
+            AvaloniaProperty.RegisterAttached<ItemsControl, Control, ControlTheme?>("HasAppliedItemContainerTheme");
+
         /// <summary>
         /// Gets or sets the <see cref="IBinding"/> to use for binding to the display member of each item.
         /// </summary>
@@ -663,12 +666,27 @@ namespace Avalonia.Controls
 
         internal void PrepareItemContainer(Control container, object? item, int index)
         {
-            var itemContainerTheme = ItemContainerTheme;
+            // If the container has no theme set, or we've already applied our ItemContainerTheme
+            // (and it hasn't changed since) then we're in control of the container's Theme and may
+            // need to update it.
+            if (!container.IsSet(ThemeProperty) || container.GetValue(AppliedItemContainerTheme) == container.Theme)
+            {
+                var itemContainerTheme = ItemContainerTheme;
 
-            if (itemContainerTheme is null)
-                container.Theme = null;
-            else if (itemContainerTheme.TargetType?.IsAssignableFrom(GetStyleKey(container)) == true)
-                container.Theme = itemContainerTheme;
+                if (itemContainerTheme?.TargetType?.IsAssignableFrom(GetStyleKey(container)) == true)
+                {
+                    // We have an ItemContainerTheme and it matches the container. Set the Theme
+                    // property, and mark the container as having had ItemContainerTheme applied.
+                    container.SetCurrentValue(ThemeProperty, itemContainerTheme);
+                    container.SetValue(AppliedItemContainerTheme, itemContainerTheme);
+                }
+                else
+                {
+                    // Otherwise clear the theme and the HasAppliedItemContainerTheme property.
+                    container.ClearValue(ThemeProperty);
+                    container.ClearValue(AppliedItemContainerTheme);
+                }
+            }
 
             if (item is not Control)
                 container.DataContext = item;

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -239,6 +239,45 @@ namespace Avalonia.Controls.UnitTests
             Assert.Null(container.Background);
         }
 
+        [Fact]
+        public void ItemContainerTheme_Should_Not_Override_LocalValue_Theme()
+        {
+            using var app = Start();
+
+            var theme1 = new ControlTheme
+            {
+                TargetType = typeof(ContentPresenter),
+                Setters = { new Setter(ContentPresenter.BackgroundProperty, Brushes.Red) }
+            };
+
+            var theme2 = new ControlTheme
+            {
+                TargetType = typeof(Control),
+                Setters = { new Setter(ContentPresenter.BackgroundProperty, Brushes.Green) }
+            };
+
+            var items = new object[]
+            {
+                new ContentPresenter(),
+                new ContentPresenter
+                {
+                    Theme = theme2
+                },
+            };
+
+            var target = CreateTarget(
+                itemsSource: items,
+                itemContainerTheme: theme1);
+
+            Assert.Same(theme1, GetContainer(target, 0).Theme);
+            Assert.Same(theme2, GetContainer(target, 1).Theme);
+
+            target.ItemContainerTheme = null;
+
+            Assert.Null(GetContainer(target, 0).Theme);
+            Assert.Same(theme2, GetContainer(target, 1).Theme);
+        }
+
         [Fact]
         public void Container_Should_Have_LogicalParent_Set_To_ItemsControl()
         {