docs: add basic theme document for Angular UI

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

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+# Angular UI: Basic Theme
+
+The Basic Theme is a theme implementation for the Angular UI. It is a minimalist theme that doesn't add any styling on top of the plain [Bootstrap](https://getbootstrap.com/). You can take the Basic Theme as the **base theme** and build your own theme or styling on top of it. See the *Customization* section.
+
+> If you are looking for a professional, enterprise ready theme, you can check the [Lepton Theme](https://commercial.abp.io/themes), which is a part of the [ABP Commercial](https://commercial.abp.io/).
+
+> See the [Theming document](Theming.md) to learn about themes.
+
+## Installation
+
+**This theme is already installed** when you create a new solution using the [startup templates](../../Startup-Templates/Index.md). If you need to manually install it, follow the steps below:
+
+* Install the [@abp/ng.theme.basic](https://www.npmjs.com/package/@abp/ng.theme.basic) NPM package to your Angular project.
+* Open the `src/app/app.module.ts` file, import `ThemeBasicModule` (it can be imported from `@abp/ng.theme.basic` package), and add `ThemeBasicModule.forRoot()` to the `imports` array.
+* Open the `src/app/shared/shared.module` file, import `ThemeBasicModule` (it can be imported from `@abp/ng.theme.basic` package), and add `ThemeBasicModule` to the `imports` and `exports` array.
+
+The `ThemeBasicModule` is registered own layouts (`ApplicationLayoutComponent`, `AccountLayoutComponent`, `EmptyLayoutComponent`) to a service which is exposed by `@abp/ng.core` package on application initialization. 
+
+## Application Layout
+
+![basic-theme-application-layout](../../images/basic-theme-application-layout.png)
+
+Application Layout implements the following parts, in addition to the common parts mentioned above;
+
+* Logo area
+* Routes area
+* Language selection & user menu
+* [Page Alerts](Page-Alerts.md)
+
+See Application Layout components:
+
+![application layout components](./images/layout-components.png)
+
+### How to Use a Layout
+
+Routes should be added to the menu by calling `add` method `RoutesService`. A layout can be set in the object of your route. See the [modifying the menu](Modifying-the-Menu#how-to-add-a-navigation-element) for more information.
+
+## Customization
+
+You have two options two customize this theme:
+
+### Overriding Styles / Components
+
+In this approach, you continue to use the theme as an NPM package and customize the parts you need to. There are several ways to customize it;
+
+#### Override the Styles
+
+You can simply override the styles in the global styles (`src/styles.scss`) file of your application.
+
+#### Override the Components
+
+See the [Component Replacement](Component-Replacement.md) to learn how you can replace components, customize and extend the user interface.
+
+### Copy & Customize
+
+You can run the following [ABP CLI](../../CLI.md) command in **Angular** project directory to copy the source code to your solution:
+
+`abp add-package @abp/ng.theme.basic --with-source-code`
+
+----
+
+Or, you can download the [source code](https://github.com/abpframework/abp/blob/dev/npm/ng-packs/packages/theme-basic) of the Basic Theme, manually copy the project content into your project (`projects/theme-basic` folder), open `angular.json` file and add configuration below to the `projects` object:
+
+```json
+{
+    "projects": {
+        ...
+        "theme-basic": {
+            "projectType": "library",
+            "root": "projects/theme-basic",
+            "sourceRoot": "projects/theme-basic/src",
+            "prefix": "abp",
+            "architect": {
+                "build": {
+                    "builder": "@angular-devkit/build-ng-packagr:build",
+                    "options": {
+                        "tsConfig": "projects/theme-basic/tsconfig.lib.json",
+                        "project": "projects/theme-basic/ng-package.json"
+                    },
+                    "configurations": {
+                        "production": {
+                        "tsConfig": "projects/theme-basic/tsconfig.lib.prod.json"
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+```
+
+Then, open the `tsconfig.json` file and add new paths as follows:
+
+```json
+"paths": {
+    ...
+    "@abp/ng.theme.basic": ["projects/theme-basic/src/public-api.ts"],
+    "@abp/ng.theme.basic/testing": ["projects/theme-basic/testing/src/public-api.ts"]
+}
+```
+
+
+You can now freely customize the theme based on your application requirements.
+
+## See Also
+
+* [Theming](Theming.md)