Paste at the bottom of Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
for a cool dark theme
document.addEventListener('DOMContentLoaded', function() {
$.ajax({
url: 'https://gist.githubusercontent.com/MitchellMalleo/7c29de8681fb8e058554dfd5c00d1715/raw/3388d93d9fad321360d56f67104fd4df4cd7bc23/black.css',
success: function(css) {
$("<style></style>").appendTo('head').html(css);
}
});