added theme mode to view and object accessor for setting properties

ThemeMode.cs

@@ -0,0 +1,11 @@
+using System.ComponentModel;
+
+namespace ModernWpfPlayground
+{
+    public enum ThemeMode
+    {
+        [Description("Light")] Light,
+        [Description("Dark")] Dark,
+        [Description("Use system setting")] UseSystemSetting
+    }
+}