The design is intended to use Numix icons. Oomox is completely useless for this as I tried to see if I couldn't use the current iteration of Numix icons (I tried), and since Numix is incredibly flat I don't think it would be an issue for the art team to fart out a hue-shifted Numix variant in GIMP or use something else they agree upon.
The FM theme in the cap isn't something I find particularly appealing, and is probably exclusive to the author. Then again I was just doing some research for myself and encountered it. and from further reading seems like something that only works in Nautilus? I am not sure. I also don't feel like testing it, and since it's from a PPA my copy is probably outdated.
If the Material / Flat look isn't what the art team wants to do, then this concept is stillborn. Apologies if the capture doesn't explain fully what the hell the theme is, if you want to fetch it you can get it here; https://www.mate-look.org/p/1220749/
My installation is from NoobsLab's theme repository for Cosmic / Bionic.