If you mean the color icons, you can easily switch back to those in the Welcome Dialog that appears when you first open GIMP (look in the Personalize tab). It's the first thing I do when I install GIMP on a new machine. :)
oh wow, i never realized that this is there, in such a convenient location too. and you can't just change the icon style, but also disable the tool groups which was the most annoying change i found because it makes finding the right tool harder. (i'd love tool groups where the tools are grouped but not folded, or in a way where i can expand certain groups that i use often)
That was a feature in GTK2 (the GUI library we use) that was removed in GTK3.
We could try to fight the library and reimplement it ourselves, but it'd take a developer dedicated to do it. I miss the menu icons too. :(
It got deprecated only in 3.10 and you can compile against that and link it with the latest Gtk+ 3 library. I do this for my programs, because I don't like it when the widget toolkit ignores my OS settings.
