Fix binding failures due to missing Background property on the base type.

ICSharpCode.ILSpyX/TreeView/SharpTreeNode.cs

@@ -134,6 +134,14 @@ namespace ICSharpCode.ILSpyX.TreeView
 			get { return null; }
 		}
 
+		public virtual object? Background {
+			get { return null; }
+		}
+
+		public virtual object? Foreground {
+			get { return null; }
+		}
+
 		public int Level {
 			get { return Parent != null ? Parent.Level + 1 : 0; }
 		}

ILSpy/ViewModels/CompareViewModel.cs

@@ -702,7 +702,7 @@ namespace ICSharpCode.ILSpy.ViewModels
 			return compareViewModel.ShowIdentical || entry.RecursiveKind != DiffKind.None ? FilterResult.Match : FilterResult.Hidden;
 		}
 
-		public Brush Background {
+		public override object? Background {
 			get {
 				switch (entry.RecursiveKind)
 				{