|  |  |  | GTK+ 3 Reference Manual |  | 
|---|
      In GtkStyle-based engines, GtkRCStyle provided ways to extend the
      gtkrc parser with engine-specific extensions. This has been replaced
      by gtk_theming_engine_register_property(), which lets a theme engine
      register new properties with an arbitrary type. While there is built-in
      support for most basic types, it is possible to use a custom parser
      for the property.
    
      The installed properties depend on the "name" property,
      so they should be added in the constructed()-Clearlooks-focus-color will be registered and
      accepted in CSS like this:
      
| 1 2 3 | GtkEntry { -Clearlooks-focus-color: rgba(255, 0, 0, 1.0); } | 
      Widget style properties also follow a similar syntax, with the widget
      type name used as a prefix. For example, the "focus-line-width"
      style property can be modified in CSS as
      -GtkWidget-focus-line-width.