Initial theming doc.

docs/en/UI/AspNetCore/Basic-Theme.md

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+# ASP.NET Core MVC / Razor Pages: The Basic Theme
+
+This document is in progress. See the [Theming](Theming.md) document.

docs/en/UI/AspNetCore/Theming.md

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-# ASP.NET Core MVC / Razor Pages Theming
+# ASP.NET Core MVC / Razor Pages: UI Theming
 
-TODO
+ABP Framework provides a complete **UI Theming** system with the following goals:
+
+* Reusable [application modules](../../Modules/Index.md) are developed **theme-independent**, so they can work with any UI theme.
+* UI theme is **decided by the final application**.
+* The theme is distributed via NuGet/NPM packages, so it is **easily upgradable**.
+* The final application can **customize** the selected theme.
+
+In order to accomplish these goals, ABP Framework;
+
+* Determines a set of **base libraries** used and adapted by all the themes. So, module and application developers can depend on and use these libraries without depending on a particular theme.
+* Provides a system that consists of [navigation menus](Navigation-Menu.md), [toolbars](Toolbars.md), [layout hooks](Layout-Hooks.md)... that is implemented by all the themes. So, the modules and the application to contribute to the layout to compose a consistent application UI.
+
+## The Current Themes
+
+Currently, two themes are **officially provided**:
+
+* The [Basic Theme](Basic-Theme.md) is the minimalist theme with the plain Bootstrap style. It is **open source and free**.
+* The [Lepton Theme](https://commercial.abp.io/themes) is a **commercial** theme developed by the core ABP team and is a part of the [ABP Commercial](https://commercial.abp.io/) license.
+
+There are also some community-driven themes for the ABP Framework (you can search on the web).
+
+## The Base Libraries
+
+All the themes must depend on the [@abp/aspnetcore.mvc.ui.theme.shared](https://www.npmjs.com/package/@abp/aspnetcore.mvc.ui.theme.shared) NPM package, so they are indirectly depending on the following libraries:
+
+* [Twitter Bootstrap](https://getbootstrap.com/) as the fundamental HTML/CSS framework.
+* [JQuery](https://jquery.com/) for DOM manipulation.
+* [DataTables.Net](https://datatables.net/) for data grids.
+* [JQuery Validation](https://jqueryvalidation.org/) for client side & [unobtrusive](https://github.com/aspnet/jquery-validation-unobtrusive) validation
+* [FontAwesome](https://fontawesome.com/) as the fundamental CSS font library.
+* [SweetAlert](https://sweetalert.js.org/) to show fancy alert message and confirmation dialogs.
+* [Toastr](https://github.com/CodeSeven/toastr) to show toast notifications.
+* [Lodesh](https://lodash.com/) as a utility library.
+* [Luxon](https://moment.github.io/luxon/) for date/time operations.
+* [JQuery Form](https://github.com/jquery-form/form) for AJAX forms.
+* [bootstrap-datepicker](https://github.com/uxsolutions/bootstrap-datepicker) to show date pickers.
+* [Select2](https://select2.org/) for better select/combo boxes.
+* [Timeago](http://timeago.yarp.com/) to show automatically updating fuzzy timestamps.
+* [malihu-custom-scrollbar-plugin](https://github.com/malihu/malihu-custom-scrollbar-plugin) for custom scrollbars.
+
+These libraries are selected as the base libraries and available to the applications and modules.
+
+### Abstractions / Wrappers
+
+There are some abstractions in the ABP Framework to make your code independent from some of these libraries too. Examples;
+
+* [Tag Helpers](Tag-Helpers/Index.md) makes it easy to generate the Bootstrap UIs.
+* JavaScript [Message](JavaScript-API/Message.md) and [Notification](JavaScript-API/Notify.md) APIs provides abstractions to use the Sweetalert and Toastr.
+* [Forms & Validation](Forms-Validation.md) system automatically handles the validation, so you mostly don't directly type any validation code.
+
+## The Layouts
+
+The main responsibility of a theme is to provide the layouts. There are **three pre-defined layouts must be supported by all the themes**:
+
+* **Application**: The default layout which is used by the main application pages.
+* **Account**: Mostly used by the [account module](../../Modules/Account.md) for login, register, forgot password... pages.
+* **Empty**: The Minimal layout that has no layout components at all.
+
+Layout names are constants defined in the `Volo.Abp.AspNetCore.Mvc.UI.Theming.StandardLayouts` class.
+
+### The Application Layout
+
+This is the default layout which is used by the main application pages. The following image shows the user management page in the [Basic Theme](Basic-Theme.md) application layout:
+
+![basic-theme-application-layout](../../images/basic-theme-application-layout.png)
+
+And the same page is shown below with the [Lepton Theme](https://commercial.abp.io/themes) application layout:
+
+![basic-theme-application-layout](../../images/lepton-theme-application-layout.png)
+
+As you can see, the page is the same, but the look is completely different in the themes above.
+
+TODO...