Prevent ControlTheme as a nested style.

src/Avalonia.Base/Styling/ControlTheme.cs

@@ -7,6 +7,17 @@ namespace Avalonia.Styling
     /// </summary>
     public class ControlTheme : StyleBase
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        public ControlTheme() { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ControlTheme"/> class.
+        /// </summary>
+        /// <param name="targetType">The value for <see cref="TargetType"/>.</param>
+        public ControlTheme(Type targetType) => TargetType = targetType;
+
         /// <summary>
         /// Gets or sets the type for which this control theme is intended.
         /// </summary>
@@ -23,5 +34,10 @@ namespace Avalonia.Styling
                 SelectorMatch.AlwaysThisType :
                 SelectorMatch.NeverThisType;
         }
+
+        internal override void SetParent(StyleBase? parent)
+        {
+            throw new InvalidOperationException("ControlThemes cannot be added as a nested style.");
+        }
     }
 }

tests/Avalonia.Base.UnitTests/Styling/ControlThemeTests.cs

@@ -0,0 +1,28 @@
+using System;
+using Avalonia.Controls;
+using Avalonia.Styling;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Styling
+{
+    public class ControlThemeTests
+    {
+        [Fact]
+        public void ControlTheme_Cannot_Be_Added_To_Style_Children()
+        {
+            var target = new ControlTheme(typeof(Button));
+            var style = new Style();
+
+            Assert.Throws<InvalidOperationException>(() => style.Children.Add(target));
+        }
+
+        [Fact]
+        public void ControlTheme_Cannot_Be_Added_To_ControlTheme_Children()
+        {
+            var target = new ControlTheme(typeof(Button));
+            var other = new ControlTheme(typeof(CheckBox));
+
+            Assert.Throws<InvalidOperationException>(() => other.Children.Add(target));
+        }
+    }
+}