SASS

I prefer this over LESS simply because it has a bigger ecosystem. (also Googling it doesn't return info about the Linux command)

npm install --save-dev sass

Tip: in your sass directory make everything a submodule (by naming the file with an _filename.scss) then create an app.scss which imports all the other submodules. Then you will get a single compressed app.css

./node_modules/sass/sass.js --watch sass:static/css --style compressed