Add code to Slack to make it use a dark theme, then kill and re-open Slack. Only adds the code if it isn't there already.

dark-slack

#!/bin/bash

# make Slack use a dark theme css next time it loads

SCRIPT="
//DARKSLACK
document.addEventListener('DOMContentLoaded', function() {
 \$.ajax({
   url: 'https://cdn.rawgit.com/laCour/slack-night-mode/master/css/raw/black.css',
   success: function(css) {
     \$('<style></style>').appendTo('head').html(css);
   }
 });
});
"
FILE=/Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js

# append the script if it isn't already there
if grep -qF -- "//DARKSLACK" $FILE; then
	echo "Slack is already dark, silly"
else
	echo "Hello darkness my old friend..."

	# append the script (pipe to grep to suppress the echo to stdout)
	echo "$SCRIPT" | sudo tee -a $FILE | grep -q fdklsajf

	# kill and run Slack
	sudo killall Slack
	open /Applications/Slack.app
fi

touch /usr/local/bin/dark-slack
chmod 755 /usr/local/bin/dark-slack

Then just run dark-slack whenever Slack updates 👍