All of the code lives in src/Reflex/TodoMVC.hs
.
style.css
is embedded into the application.
You probably want to use https://nixcache.reflex-frp.org, the official build artifact cache, so you don't spend hours building dependencies from source when you run the commands below. See https://github.com/reflex-frp/reflex-platform for details.
nix-build -E '(import dep/reflex-platform {}).ghcjs.callPackage ./. {}'
cd result/bin/reflex-todomvc.jsexe
Then open index.html
in your browser!