To find dark mode in Slack on Mac, click on your name in the left sidebar > Preferences > Themes. This will show you a quick preview of how everything will look, and then you can choose to switch to dark mode or stay in light mode.
The company has also added new themes specifically tailored for dark mode, all of which are also available today. There is no required update to see dark mode on Slack, but you'll need to restart the app to get it to appear.
Slack also confirmed that in the near future you will be able to set Slack to switch automatically between light and dark mode, based on your macOS settings. The company didn't provide a specific release date for this feature but said it will be available fairly soon.