Give yourself a black background, with grey text and red names. Works with Slack 4.0 on OSX.
Clone this repo.
git clone https://github.com/tnielsen2/slack-color-theme.git .
Make sure you have node and npm installed via brew.
brew install npm
brew install node
Modify the hex code of your colors in the colors.js file to whatever you desire. Save the file.
Run this script as SU.
sudo bash ./osx.sh
Restart Slack.