To start, always make sure your file is clean and not modified. Paste this into your command prompt:
osascript -e 'quit app "slack"';curl 'https://raw.githubusercontent.com/valiander/slackdark/master/reset' > /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
This command will quit slack and overwrite the file needed to change your css entries.
This will also return your slack app back to normal, aside from the theme changes.
To set the dark theme, paste this into your command prompt:
osascript -e 'quit app "slack"';curl 'https://raw.githubusercontent.com/valiander/slackdark/master/darktheme' >> /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
This is going to quit slack and append the changes to the end of the file.
Its important to quit and restart slack every time we change this file so that the changes take effect.
Last step is to paste this into a chat with yourself or slackbot, and click "switch sidebar theme"
#3b383b,#b8cbe6,#ffffff,#ffffff,#4A5664,#ffffff,#00f7ff,#3b383b
If you see the following error while trying to run this, delete slack and reinstall.
-bash: /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js: Permission denied
Let me know if there are any issues that you have!