If you haven't already tweaked the app yourself, dedicated dark mode themes are now available on Slack's Mac, Windows and Linux desktop clients. They join the Android and iOS mobile apps where it was already a feature. You can either apply various dark mode sidebar variants, or try the new "Nocturne" dark mode theme, which is now even darker.
George Zamfir, Slack's Accessibility Product Manager, says the dark mode is "helpful for working at night or in low-light" and it's important for enabling accessibility for people with "visual impairments, migraines or other visual disorders".
To enable dark mode in the Slack desktop client, go to Preferences and then to Themes. Here you can preview different sidebar options or toggle between light and dark modes. In the future, Slack says it will enable an option for its client to sync with your desktop OS's dark mode preferences as well.