Slack themes are available in the web app using Stylish see https://userstyles.org/styles/browse?search_terms=slack
However there must be a way to use them on the Desktop application. What is the hack?
UPDATED LATEST! Slack desktop app now supports dark mode natively!
Just got to preferences cmd+,
and select Themes > Dark
UPDATED, previous hacks stopped working with release of 4.0.0.
This solution works as of July 18, 2019
see https://github.com/LanikSJ/slack-dark-mode
You may need to see instructions on https://github.com/LanikSJ/slack-dark-mode/issues/80
I will likely update this answer again when I have time to fork the repo I've posted above and improve upon it.