Update: Slack now has dark theme integrated in the desktop app. The slack black project and this helper script are no longer needed.
Windows Powershell script to apply slack-black-theme on Slack v4.0
You must have 7zip and the ASAR addin (http://www.tc4shell.com/binary/Asar.zip) installed
Download script and run For a one-time bypass on execution policy, use this command line: PowerShell.exe -ExecutionPolicy Bypass -File .slackBlack.ps1
Let me know of any errors you have