Problem:
One can not customize parts of GTK, since the widget and class paths are
unknown.

Please look at the attached code.
It creates a main window, a menubar and one menu.  Also, it creates a
GtkFileSelection when "Open..." is selected from the menu.

The names set for widgets are:
GtkWindow(menutest).GtkVBox(vbox).GtkMenuBar(menubar)

Also,
GtkMenu(filemenu).GtkMenuItem(menuitem_1 and menuitem_2).

GtkFileSelection(file_dialog).

To change style for the menu, I assumed this would work:
widget "menutest.vbox.menubar*" style "my_style"

Alas, it does not change style of the menu items, only the menu bar itself.

Even this does not work:
widget "menutest*menuitem*" style "my_style"

But this does:
widget "*menuitem*" style "my_style"

but now I have lost the information that only menu items under "menutest"
shall be customized.  I could rename my menu items "menutest_menuitem_n",
so then ""menutest*menuitem*" would work.  But this is not a general solution.

I can not say "Apply this style to everything created by menutest" in
principle like this (fixing the names), because some names are unknown. 
Take the file selection dialog.  I would like to customize that.  I name it
"file_dialog", so I can say
widget "*file_dialog*" style "my_style"

But again, we loose the information that we really only mean file dialogs
created by "menutest".  Also, this does not customize the option menu or
the dialogs created by file selection itself, for example, when pressing
the "New Folder" there is a dialog popped up, this is not affected by
widget "*file_dialog*"

In fact, there is no documented way at all to customize a file dialog or
menus created by one application, since the file dialog names used aren't
documented.  This does not customize menus or dialogs used by a file dialog
either:
widget_class "*GtkFileSelection*" style "my_style"

Contrast this to Xt, where I can say (for a Motif compiled emacs)
emacs*background: red

and all widgets, menus, dialogs use red as background.

I'd like to see something similar in GTK, so that menus and dialogs pick up
the path names from their parents.  They don't seem to do so now.