Added ItemContainerTheme property.

Otherwise there's no easy way to apply themes to item containers.

src/Avalonia.Controls/Generators/IItemContainerGenerator.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using Avalonia.Controls.Templates;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls.Generators
 {
@@ -14,6 +15,11 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         IEnumerable<ItemContainerInfo> Containers { get; }
 
+        /// <summary>
+        /// Gets or sets the theme to be applied to the items in the control.
+        /// </summary>
+        ControlTheme? ItemContainerTheme { get; set; }
+
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>

src/Avalonia.Controls/Generators/ItemContainerGenerator.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls.Generators
 {
@@ -35,6 +36,11 @@ namespace Avalonia.Controls.Generators
         /// <inheritdoc/>
         public event EventHandler<ItemContainerEventArgs>? Recycled;
 
+        /// <summary>
+        /// Gets or sets the theme to be applied to the items in the control.
+        /// </summary>
+        public ControlTheme? ItemContainerTheme { get; set; }
+
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>
@@ -190,10 +196,18 @@ namespace Avalonia.Controls.Generators
                     result.SetValue(
                         ContentPresenter.ContentTemplateProperty,
                         ItemTemplate,
-                        BindingPriority.TemplatedParent);
+                        BindingPriority.Style);
                 }
             }
 
+            if (ItemContainerTheme != null)
+            {
+                result.SetValue(
+                    StyledElement.ThemeProperty,
+                    ItemContainerTheme,
+                    BindingPriority.TemplatedParent);
+            }
+
             return result;
         }
 

src/Avalonia.Controls/Generators/ItemContainerGenerator`1.cs

@@ -44,28 +44,29 @@ namespace Avalonia.Controls.Generators
         {
             var container = item as T;
 
-            if (container != null)
+            if (container is null)
             {
-                return container;
-            }
-            else
-            {
-                var result = new T();
+                container = new T();
 
                 if (ContentTemplateProperty != null)
                 {
-                    result.SetValue(ContentTemplateProperty, ItemTemplate, BindingPriority.Style);
+                    container.SetValue(ContentTemplateProperty, ItemTemplate, BindingPriority.Style);
                 }
 
-                result.SetValue(ContentProperty, item, BindingPriority.Style);
+                container.SetValue(ContentProperty, item, BindingPriority.Style);
 
                 if (!(item is IControl))
                 {
-                    result.DataContext = item;
+                    container.DataContext = item;
                 }
+            }
 
-                return result;
+            if (ItemContainerTheme != null)
+            {
+                container.SetValue(StyledElement.ThemeProperty, ItemContainerTheme, BindingPriority.Style);
             }
+
+            return container;
         }
 
         /// <inheritdoc/>

src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs

@@ -71,6 +71,11 @@ namespace Avalonia.Controls.Generators
                 var template = GetTreeDataTemplate(item, ItemTemplate);
                 var result = new T();
 
+                if (ItemContainerTheme != null)
+                {
+                    result.SetValue(Control.ThemeProperty, ItemContainerTheme, BindingPriority.Style);
+                }
+
                 result.SetValue(ContentProperty, template.Build(item), BindingPriority.Style);
 
                 var itemsSelector = template.ItemsSelector(item);

src/Avalonia.Controls/ItemsControl.cs

@@ -15,6 +15,7 @@ using Avalonia.Input;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
 using Avalonia.VisualTree;
+using Avalonia.Styling;
 
 namespace Avalonia.Controls
 {
@@ -36,6 +37,12 @@ namespace Avalonia.Controls
         public static readonly DirectProperty<ItemsControl, IEnumerable?> ItemsProperty =
             AvaloniaProperty.RegisterDirect<ItemsControl, IEnumerable?>(nameof(Items), o => o.Items, (o, v) => o.Items = v);
 
+        /// <summary>
+        /// Defines the <see cref="ItemContainerTheme"/> property.
+        /// </summary>
+        public static readonly StyledProperty<ControlTheme?> ItemContainerThemeProperty =
+            AvaloniaProperty.Register<ItemsControl, ControlTheme?>(nameof(ItemContainerTheme));
+
         /// <summary>
         /// Defines the <see cref="ItemCount"/> property.
         /// </summary>
@@ -88,6 +95,7 @@ namespace Avalonia.Controls
                 {
                     _itemContainerGenerator = CreateItemContainerGenerator();
 
+                    _itemContainerGenerator.ItemContainerTheme = ItemContainerTheme;
                     _itemContainerGenerator.ItemTemplate = ItemTemplate;
                     _itemContainerGenerator.Materialized += (_, e) => OnContainersMaterialized(e);
                     _itemContainerGenerator.Dematerialized += (_, e) => OnContainersDematerialized(e);
@@ -108,6 +116,15 @@ namespace Avalonia.Controls
             set { SetAndRaise(ItemsProperty, ref _items, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the <see cref="ControlTheme"/> that is applied to the container element generated for each item.
+        /// </summary>
+        public ControlTheme? ItemContainerTheme
+        {
+            get { return GetValue(ItemContainerThemeProperty); }
+            set { SetValue(ItemContainerThemeProperty, value); }
+        }
+
         /// <summary>
         /// Gets the number of items in <see cref="Items"/>.
         /// </summary>
@@ -349,6 +366,10 @@ namespace Avalonia.Controls
             {
                 UpdatePseudoClasses(change.GetNewValue<int>());
             }
+            else if (change.Property == ItemContainerThemeProperty && _itemContainerGenerator is not null)
+            {
+                _itemContainerGenerator.ItemContainerTheme = change.GetNewValue<ControlTheme?>();
+            }
         }
 
         /// <summary>

src/Avalonia.Controls/TreeViewItem.cs

@@ -121,6 +121,11 @@ namespace Avalonia.Controls
             {
                 ItemTemplate = _treeView.ItemTemplate;
             }
+
+            if (ItemContainerTheme == null && _treeView?.ItemContainerTheme != null)
+            {
+                ItemContainerTheme = _treeView.ItemContainerTheme;
+            }
         }
 
         protected override void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)

tests/Avalonia.Controls.UnitTests/ItemsControlTests.cs

@@ -1,15 +1,14 @@
+using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
+using Avalonia.Input;
 using Avalonia.LogicalTree;
-using Avalonia.VisualTree;
-using Xunit;
-using System.Collections.ObjectModel;
+using Avalonia.Styling;
 using Avalonia.UnitTests;
-using Avalonia.Input;
-using System.Collections.Generic;
+using Xunit;
 
 namespace Avalonia.Controls.UnitTests
 {
@@ -62,6 +61,25 @@ namespace Avalonia.Controls.UnitTests
             Assert.Null(container.TemplatedParent);
         }
 
+        [Fact]
+        public void Container_Should_Have_Theme_Set_To_ItemContainerTheme()
+        {
+            var theme = new ControlTheme();
+            var target = new ItemsControl
+            {
+                ItemContainerTheme = theme,
+            };
+
+            target.Template = GetTemplate();
+            target.Items = new[] { "Foo" };
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var container = (ContentPresenter)target.Presenter.Panel.Children[0];
+
+            Assert.Same(container.Theme, theme);
+        }
+
         [Fact]
         public void Container_Should_Have_LogicalParent_Set_To_ItemsControl()
         {

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -94,6 +94,50 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
+        [Fact]
+        public void Container_Should_Have_Theme_Set_To_ItemContainerTheme()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var items = new[] { "Foo", "Bar", "Baz " };
+                var theme = new ControlTheme();
+                var target = new ListBox
+                {
+                    Template = ListBoxTemplate(),
+                    Items = items,
+                    ItemContainerTheme = theme,
+                };
+
+                Prepare(target);
+
+                var container = (ListBoxItem)target.Presenter.Panel.Children[0];
+
+                Assert.Same(container.Theme, theme);
+            }
+        }
+
+        [Fact]
+        public void Inline_Item_Should_Have_Theme_Set_To_ItemContainerTheme()
+        {
+            using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
+            {
+                var items = new[] { "Foo", "Bar", "Baz " };
+                var theme = new ControlTheme();
+                var target = new ListBox
+                {
+                    Template = ListBoxTemplate(),
+                    Items = new[] { new ListBoxItem() },
+                    ItemContainerTheme = theme,
+                };
+
+                Prepare(target);
+
+                var container = (ListBoxItem)target.Presenter.Panel.Children[0];
+
+                Assert.Same(container.Theme, theme);
+            }
+        }
+
         [Fact]
         public void LogicalChildren_Should_Be_Set_For_DataTemplate_Generated_Items()
         {

tests/Avalonia.Controls.UnitTests/TreeViewTests.cs

@@ -72,6 +72,35 @@ namespace Avalonia.Controls.UnitTests
             Assert.All(items, x => Assert.IsType<Canvas>(x.HeaderPresenter.Child));
         }
 
+        [Fact]
+        public void Items_Should_Be_Created_Using_ItemConatinerTheme_If_Present()
+        {
+            TreeView target;
+            var theme = new ControlTheme();
+
+            var root = new TestRoot
+            {
+                Child = target = new TreeView
+                {
+                    Template = CreateTreeViewTemplate(),
+                    Items = CreateTestTreeData(),
+                    ItemContainerTheme = theme,
+                    ItemTemplate = new FuncTreeDataTemplate<Node>(
+                        (_, __) => new Canvas(),
+                        x => x.Children),
+                }
+            };
+
+            ApplyTemplates(target);
+
+            var items = target.ItemContainerGenerator.Index.Containers
+                .OfType<TreeViewItem>()
+                .ToList();
+
+            Assert.Equal(5, items.Count);
+            Assert.All(items, x => Assert.Same(theme, x.ItemContainerTheme));
+        }
+
         [Fact]
         public void Root_ItemContainerGenerator_Containers_Should_Be_Root_Containers()
         {