Sblack is a little free Mac app that applies a Dark Theme to Slack for Mac to make it easier on your eyes. Just drag and drop and you're done!
Stylus is a fork of Stylish that can be used to redesign the web. The objective in creating Stylus was to remove any and all analytics, and return to a more user-friendly UI. This is why we named the extension with an "us" ending, as in the actual users.