## Extending the CSS parser

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 This has been replaced by `gtk_theming_engine_register_property()`, which lets a theme engine register new properties with an arbitrary type.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()`` vfunc. For example, if an engine with the name "Clearlooks" installs a "focus-color" property with the type GdkRGBA, the property `-Clearlooks-focus-color` will be registered and accepted in CSS like this:

          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 GtkWidget focus-line-width style property can be modified in CSS as `-GtkWidget-focus-line-width`.