If you are already signed up for the beta program, you just need to make sure that you have updated to the latest version of the app to gain access to the dark mode option. From there, you can enable the dark mode in the settings. To apply, head over to the Slack for Android beta page on the official Slack website. While the desktop app has not received any such similar update, the Slack team promises that the desktop app will get the dark mode in the future as well. Then you'll see a dark mode option under the "General" tab of your settings menu.
If required, restart the Slack app. Per 9to5Google, the Dark Mode covers the majority of the app-including in-app menus and pop-ups-though the site did note that it doesn't appear to touch "the color palette of the Slack room". It takes a lot of work to get dark mode in Facebook Messenger since you have to send yourself or someone else the moon emoji.
Image Stan Schroeder Mashable
As noted above, the feature is now limited to just the beta users. The dark mode background since it is either black or grey provides you with a much longer battery life while using that app. The new dark mode is only available to those who have signed up as a beta tester for Slack, so if you haven't done that yet now would be a good time.
We can't go through every single app for Android and iOS with a dark mode, but those are the main ones. But why not darken your apps where you can in the meantime?