[C] Cascade StyleSheets defined at Application level (#1590)

fixes #1553

Xamarin.Forms.Core.UnitTests/StyleSheets/StyleTests.cs

@@ -17,6 +17,13 @@ namespace Xamarin.Forms.StyleSheets.UnitTests
 			Internals.Registrar.RegisterAll(new Type[0]);
 		}
 
+		[TearDown]
+		public void TearDown()
+		{
+			Device.PlatformServices = null;
+			Application.ClearCurrent();
+		}
+
 		[Test]
 		public void PropertiesAreApplied()
 		{
@@ -78,5 +85,17 @@ namespace Xamarin.Forms.StyleSheets.UnitTests
 			var layout = new StackLayout();
 			Assert.That(layout.GetValue(TextElement.TextColorProperty), Is.EqualTo(Color.Default));
 		}
+
+		[Test]
+		public void StyleSheetsOnAppAreApplied()
+		{
+			var app = new MockApplication();
+			app.Resources.Add(StyleSheet.FromString("label{ color: red;}"));
+			var page = new ContentPage {
+				Content = new Label()
+			};
+			app.MainPage = page;
+			Assert.That((page.Content as Label).TextColor, Is.EqualTo(Color.Red));
+		}
 	}
 }

Xamarin.Forms.Core/Element_StyleSheets.cs

@@ -41,8 +41,8 @@ namespace Xamarin.Forms
 				return;
 			var sheets = new List<StyleSheet>();
 			while (parent != null) {
-				var visualParent = parent as VisualElement;
+				var resourceProvider = parent as IResourcesProvider;
-				var vpSheets = visualParent?.GetStyleSheets();
+				var vpSheets = resourceProvider?.GetStyleSheets();
 				if (vpSheets != null)
 					sheets.AddRange(vpSheets);
 				parent = parent.Parent;