An open source version that is not affiliated with f.lux:
I don't know which license matches this so I'm going to write it out. You can do what you want with the code, but do not distribute compiled copies of the app, especially on mass download sites.
I mean, it's not quite BSD, but it's hardly 'all rights reserved'.
// Created by Thomas Finch on 6/22/15.
// Copyright (c) 2015 Thomas Finch. All rights reserved.(Though it looks like this project doesn't have a standard license)