Slack-Dark-Mode

Slack Night Mode by laCour theme reposted in a css version compatible with Slack for Desktop.

Append to your Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js the following code:

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'FILE_URL',
   success: function(css) {
     $("<style></style>").appendTo('head').html(css);
   }
 });
});

and replace FILE_URL with your file url path (e.g. for me is https://cdn.rawgit.com/mattiacantalu/Slack-Dark-Mode/master/dark-mode.css).