GtkMenubar.shadow-type and other style properties are read only. The only way you can change it is by passing in new rc strings. Something like this should do it:

    style "menubar-style" { 
        GtkMenuBar::shadow_type = etched-in 
    }

    class "GtkMenuBar" style "menubar-style"

In this case, possible shadow_types are:

  none, in, out, etched-in, etched-out

In general to find out what you can parse you have to check the enum, eg: [pygtk.org] s

Which can be passed on to gtk.rc_parse_string (or put in a file and then passed to gtk.rc_parse).