This repo is inteded for us with MacOS only.
In order to automate the intallation, just clone the repo and run the bash script:
bash turn_slack_dark.sh
Be aware that gnu-sed is required and it will attempt to install it using brew if you don't have it.
CSS theme copied from: https://cdn.rawgit.com/laCour/slack-night-mode/master/css/raw/black.css