Make styles inherited by default

Ooui/Style.cs

@@ -17,12 +17,17 @@ namespace Ooui
             set => SetProperty ("background-color", value);
         }
 
+        public Value BackgroundImage {
+            get => GetProperty ("background-image");
+            set => SetProperty ("background-image", value);
+        }
+
         private Value GetProperty (string propertyName)
         {
             lock (properties) {
                 Value p;
                 if (!properties.TryGetValue (propertyName, out p)) {
-                    p = new Value ();
+                    p = "inherit";
                     properties[propertyName] = p;
                 }
                 return p;
@@ -31,13 +36,14 @@ namespace Ooui
 
         private void SetProperty (string propertyName, Value value)
         {
+            var safeValue = value ?? "inherit";
             lock (properties) {
                 Value old;
                 if (properties.TryGetValue (propertyName, out old)) {
-                    if (EqualityComparer<Value>.Default.Equals (old, value))
+                    if (EqualityComparer<Value>.Default.Equals (old, safeValue))
                         return;
                 }
-                properties[propertyName] = value;
+                properties[propertyName] = safeValue;
             }
             PropertyChanged?.Invoke (this, new PropertyChangedEventArgs (propertyName));
         }

Tests/StyleTests.cs

@@ -8,6 +8,23 @@ namespace Tests
     [TestClass]
     public class StyleTests
     {
+        [TestMethod]
+        public void DefaultIsInherit ()
+        {
+            var s = new Style ();
+            Assert.AreEqual ("inherit", s.BackgroundColor);
+        }
+
+        [TestMethod]
+        public void NullMakesInherit ()
+        {
+            var s = new Style ();
+            s.BackgroundColor = "red";
+            Assert.AreEqual ("red", s.BackgroundColor);
+            s.BackgroundColor = null;
+            Assert.AreEqual ("inherit", s.BackgroundColor);
+        }
+
         [TestMethod]
         public void ChangedWhen ()
         {