Finds all matching RC styles for a given widget,
composites them together, and then creates a
GtkStyle representing the composite appearance.
(GTK+ actually keeps a cache of previously
created styles, so a new style may not be
GtkStyle] the resulting style. No refcount is added
to the returned style, so if you want to save this
style around, you should add a reference yourself.
Derived by RGtkGen from GTK+ documentation