modified theming.md

docs/theming.md

@@ -24,10 +24,11 @@ To define a css theme for Element:
 In future, the assets for a theme will probably be gathered together into a
 single directory tree.
 
- JSON Based Themes (Labs Feature):
-==================================
+ JSON Based Themes
+==================
+
+JSON themes derived from the built in CSS themes may be defined in the `config.json` file. When [`feature_custom_themes`](https://github.com/vector-im/element-web/blob/master/docs/labs.md#custom-themes-feature_custom_themes) is enabled, users can add new JSON themes in the Appearance section of Settings.
 
-Themes derived from the built in themes may also be defined in settings.
 
 To avoid name collisions, the internal name of a theme is
 `custom-${theme.name}`. So if you want to set the custom theme below as the