It is a Stylish theme for Slack in the browser.<p>Here is a link to the Github project: <a href="https:&#x2F;&#x2F;github.com&#x2F;laCour&#x2F;slack-night-mode" rel="nofollow">https:&#x2F;&#x2F;github.com&#x2F;laCour&#x2F;slack-night-mode</a><p>I wish the Slack App had a night theme..