feat(demo): add landing page with docs (#1951)

docs/articles/theme-change.md

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+# Change Current Theme
+
+Nebular Theme System provides 3 color schemes out of the box - `default`, `corporate` and `cosmic`. It is both possible to change the theme statically and dynamically during the runtime.
+
+
+## Switch from Cosmic to Default
+It is extremely simple to replace a theme from one to another.
+All you need to do is to find your `NbThemeModule.forRoot` declaration and change a value of the `name` setting:
+
+```ts
+  @NgModule({
+    imports: [
+      // ...
+      NbThemeModule.forRoot({ name: 'default' }),
+    ],
+  }
+```
+<hr>
+
+## Runtime Theme Switch
+In case you want to have a better control when a theme is changed, or for instance need to change it based on a user role,
+it is possible to dynamically tell Nebular which theme should be enabled. 
+`NbThemeService` is our friend in this case and particularly the `changeTheme` method:
+
+```ts
+
+  // ...
+  constructor(private themeService: NbThemeService) {
+    this.themeService.changeTheme('corporate');
+  }
+
+```
+<hr>
+
+## Listen to Theme Change
+And of course it is possible to subscribe to an event when the current theme gets changed so that you can adjust something in your code accordingly:
+
+```ts
+
+  // ...
+  constructor(private themeService: NbThemeService) {
+  
+    this.themeService.onThemeChange()
+          .subscribe((theme: any) => {
+            console.log(`Theme changed to ${theme.name}`);
+          });
+  }
+
+```
+<hr>
+
+## Related Articles
+
+- [Theme System](docs/guides/theme-system)