SlackTheme

For Windows:

go to AppData\Local\slack\app-3.3.1\resources\app.asar.unpacked\src\static ( if you open a folder and paste that link after your username it will take you straight there )
Right click and edit ssb-interop.js file
Paste the following code at the bottom:

document.addEventListener('DOMContentLoaded', function() {
  $.ajax({
    url: 'https://cdn.rawgit.com/TheSpitefulOctopus/SlackTheme/9903b00f/customDark.css',
           success: function(css) {
       $("<style></style>").appendTo('head').html(css);
     }
   });
});

Save && Restart Slack

For Mac Users:

Go to your Applications folder
Right click and choose "Show Package Contents"
Navigate into Contents -> Resources -> app.asar.unpacked -> src -> static
Right click and edit ssb-interop.js
Paste the following code at the bottom:

document.addEventListener('DOMContentLoaded', function() {
  $.ajax({
    url: 'https://cdn.rawgit.com/TheSpitefulOctopus/SlackTheme/9903b00f/customDark.css',
           success: function(css) {
       $("<style></style>").appendTo('head').html(css);
     }
   });
});

Save && Restart Slack