Change Skin -> Theme

To better align with commonly used terminology in industry renamed all references from Skin -> Theme.

Oqtane.Client/Themes/ThemeObjectBase.cs

@@ -0,0 +1,11 @@
+using Microsoft.AspNetCore.Components;
+using Oqtane.Shared;
+
+namespace Oqtane.Themes
+{
+    public class ThemeObjectBase : ComponentBase
+    {
+        [CascadingParameter]
+        protected PageState PageState { get; set; }
+    }
+}