Widget theming: GtkWidget vs GtkStyleContext

[Lukas Middelberg <lukas middelberg-bi net>]

Hey guys,

I am kind of confused by the way Gtk handles widget theming. I know 
that styling information is stored
in a GtkStyleContext object corresponding to the GtkWidget to be 
themed. However, if I want to install a
new style property into a widget, I need to use methods defined in 
GtkWidget:

Gtk.Widget.install_style_property ()
Gtk.Widget.install_style_property_parser ()

Also, the methods for retrieving properties seem kind of redundant -

StyleContext                                   Widget
============                                   ======

Gtk.StyleContext.get_style_property ()   vs    
Gtk.Widget.style_get_property ()
Gtk.StyleContext.get_style_valist ()     vs    
Gtk.Widget.style_get_valist ()
Gtk.StyleContext.get_valist ()
Gtk.StyleContext.@get ()                 vs    Gtk.Widget.style_get ()
 (at least that's the vala syntax)
                                               Gtk.Widget.find_style_property ()
                                              
Gtk.Widget.list_style_properties ()

Why is that? In what ways do e.g. StyleContext.get_style_property and 
Widget.style_get_property differ?
Which one of them should I use in order to be able to theme custom 
widgets?

Also, please consider this example:

If I subclass GtkWidget and register a custom property "dummy", I can 
access that property using

   myobj.style_get ("dummy", out somevariable);

whereas this fails saying "Style property 'dummy' is not registered":

   myobj.get_style_context ().@get (Gtk.StateFlags.NORMAL, "dummy", 
out somevariable);

(I put the full example into a pastebin: http://sprunge.us/BQKP)

In what ways are the StyleContext methods different from the GtkWidget 
ones? How come I can access custom-defined
style properties from GtkWidget but not from GtkStyleContext?

Can someone please clear me up?
Thanks a ton,

Lukas