Fixing #2556: finished theming docs, emphasized on the fact that themes are…

docs/guide/theming.md

 Theming
 =======
 
-TBD
+A theme is a directory of view and layout files. Each file of the theme overrides corresponding file of an application
+when rendered. A single application may use multiple themes and each may provide totally different experience. At any
+time only one theme can be active.
+
+> Note: Themes usually do not meant to be redistributed since views are too application specific. If you want to
+  redistribute customized look and feel consider CSS and JavaScript files in form of [asset bundles](assets.md) instead.
 
 Configuring current theme
 -------------------------
@@ -18,4 +23,27 @@ be in your application config file:
 		],
 	],
 ],
-```
\ No newline at end of file
+```
+
+In the above `pathMap` defines where to look for view files while `baseUrl` defines base URL for resources referenced
+from these files. For example, if `pathMap` is `['/web/views' => '/web/themes/basic']`,  then the themed version
+for a view file `/web/views/site/index.php` will be `/web/themes/basic/site/index.php`.
+
+Using multiple paths
+--------------------
+
+It is possible to map a single path to multiple paths. For example,
+
+```php
+'pathMap' => [
+	'/web/views' => [
+		'/web/themes/christmas',
+		'/web/themes/basic',
+	],
+]
+```
+
+In this case, the view will be searched in `/web/themes/christmas/site/index.php` then if it's not found it will check
+`/web/themes/basic/site/index.php`. If there's no view there as well application view will be used.
+
+This ability is especially useful if you want to temporary or conditionally override some views.