As of today (03/11/19), Slack is offering a dark or night mode to beta testers. iOS users can join the beta testing program at this link by submitting their full name and email. It typically takes a few hours to receive an invite, but some users have reported receiving an email within minutes (around 10). You can also navigate to Settings > Join Beta within the app to join.
After you're admitted to the program, you'll need to update the Slack app to the beta version. Once updated, you'll be able to enable Dark Mode in the settings.
A lot of mobile users prefer dark mode because it is easier on the eyes. It is also good for reading at night, and it is a lot easier on your device's battery.