Install less CSS and minify plugin:
npm install less -g
npm install -g less-plugin-clean-css
Compile styles:
cd webroot/ngax
lessc less/dark.less styles/dark.css --clean-css -m=always
lessc less/default.less styles/default.css --clean-css -m=always
There are warnings about math=always, but to fix those all divisions in .less need to be wrapped in parens.