Added note about configuring theme do docs

docs/guide/theming.md

 Theming
 =======
 
-TBD
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+TBD
+
+Configuring current theme
+-------------------------
+
+Theme configuration is specified via `view` component of the application. So in order to set it up the following should
+be in your application config file:
+
+```php
+'components' => [
+	'view' => [
+		'theme' => [
+			'pathMap' => ['@app/views' => '@webroot/themes/basic'],
+			'baseUrl' => '@web/themes/basic',
+		],
+	],
+],
+```
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