Yes, a real dark theme for Slack on OSX and Linux. But, you have to build it yourself. It's so easy!
This is great for those of us with bleeding eyes, the only downfall is that since we are wrapping the web app you will lose the organization switcher on the left side. However, you can still easily sign in to multiple organizations and switch using command-K.
git clone https://github.com/johnthepink/dark-slack.git cd dark-slack npm install -g nativefier nativefier https://[your-org].slack.com --inject darkslack.css --name "Dark Slack" --icon slack.icns
Now you should have a new folder with Dark Slack inside of it.