As mentioned in the README, Numix Core depends on Python 3.x, either Cairo or Inkscape for PNG exports, and libicns (specifically png2icns) for macOS packaging. This page details how to get those installed on various platforms.

# Designing icons and Converting SVGs to PNGs using Inkscape
apt install inkscape
# Converting SVGs to PNGs using Cairo
pip3 install cairosvg pygobject
# Testing dependencies
pip3 install flake8 jsonschema
# macOS packaging dependencies
apt install libicns1 icnsutils

While building themes using Core is only officially supported on Linux, @BKrajancic has kindly done some testing around running it on macOS too. The dependencies can be installed using Homebrew and pip as follows.

brew install librsvg
brew install pkg-config
pip3 install pygobject
brew install inkscape