This icon theme I found is an awesome retro icon theme. This has icons that look like the CDE theme. This would be awesome to use with a retro Gnome theme like this one: https://www.pling.com/p/1749681/ I think these icons look very good when using MATE. Coupled with the Industrial theme, this is a good combination. Very suitable for use with MATE or Cinnamon. And there are plenty of retro themes to use with this. This theme as well would be perfect: https://www.gnome-look.org/p/1231025. CDE / Motif theme for Xfce / Gtk. This would really complete a retro desktop look.
Below is what it looks like.
Download this theme: https://www.securitronlinux.com/arma3/NsCDE-NineIcons.tar.