Packaging my own GTK theme

Hey, I want to install a gtk theme that is not available in nix pkgs with home-manager, the GTK theme that I want to install is the mountain theme: GitHub - mountain-theme/Mountain: Neutral. Calm.

What should I put in my home.nix, and or should I create other files, where? what do I put in them, pls help.

See How to set up a GTK3 theme, thats not in the nix pkgs?