Might there be any risk that other themes may not work well with the changed icon references? I guess this is hard to answer; the icon loading mechanism are big mystique to me and so I'm just wondering how much we could break with this change.
Just speculating but `gtk_image_new_from_stock()` has been deprecated since GTK 3.10 and we now require GTK 3.24 so we shouldn't run into the risk that Geany is started on some outdated distribution with an icon theme not supporting the names used by `gtk_image_new_from_icon_name()`. It will probably be the other way round - that themes support the `-symbolic` names and neglect the old naming like in the case of Adwaita.