Add Themes Index to docs

docs/en/Themes/Index.md

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+# The Official Themes
+ABP Framework provides a complete UI theming system. While you can build your own themes, you can use the following pre-built themes freely in your applications.
+
+## The Basic Theme
+The Basic Theme is a minimalist theme that doesn't add any styling on top of the plain [Bootstrap](https://getbootstrap.com/) styles. You can take the Basic Theme as the base theme and build your own theme or styling on top of it. Here, a screenshot from the theme:
+
+![basic-theme-application-layout](../images/basic-theme-application-layout.png)
+
+### Documentation
+
+- [Basic Theme - MVC UI](../UI/AspNetCore/Basic-Theme.md)
+- [Basic Theme - Blazor UI](../UI/Blazor/Basic-Theme.md)
+- [Basic Theme - Angular UI](../UI/Angular/Basic-Theme.md)
+
+## The LeptonX Lite Theme
+**LeptonX Lite** is the free version of the [LeptonX Theme](https://x.leptontheme.com/), which is a part of the ABP Commercial. Here, a screenshot from the theme:
+
+![LeptonX Lite application layout](../images/leptonxlite-theme-application-layout.jpeg)
+
+### Documentation
+
+- [LeptonX Lite - MVC UI](LeptonXLite/AspNetCore.md)
+- [LeptonX Lite - Blazor UI](LeptonXLite/Blazor.md)
+- [LeptonX Lite - Angular UI](LeptonXLite/Angular.md)
+
+## See Also
+
+* [Theming - MVC UI](../UI/AspNetCore/Theming.md)
+* [Theming - Blazor UI](../UI/Blazor/Theming.md)
+* [Theming - Angular UI](../UI/Angular/Theming.md)

docs/en/docs-nav.json

@@ -1194,6 +1194,10 @@
         {
           "text": "Common",
           "items": [
+            {
+              "text": "Themes",
+              "path": "Themes/Index.md"
+            },
             {
               "text": "Overriding the User Interface",
               "path": "Customizing-Application-Modules-Overriding-User-Interface.md"