How to get Gtk::Stock::APPLY icon back in 3.12?

[Stefan Salewski <mail ssalewski de>]

I have used the green checkmark icon button up to 3.10 release.
(in toolbar,http://www.ssalewski.de/PetEd.html.en )

Since 3.12 it is not available any more, for a fast fix I replaced

# minor_grid_select = Gtk::RadioToolButton.new(@major_grid_select, Gtk::Stock::APPLY)

with these two lines

@minor_grid_select = Gtk::RadioToolButton.new(@major_grid_select)
@minor_grid_select.set_icon_name("list-add")
 
So it works again, but now I have a blue "cross" icon. I am not an
artist, so I do not want to draw my own icons. Is there an easy way to
get the green checkmark back?