Copied this code from laCour/slack-night-mode to fix an annoying issue with low contrast text in menus.
Append the following snippet to any file that Slack will interpret inside it's WebApp, for example /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
.
document.addEventListener('DOMContentLoaded', function() {
$.ajax({
url: 'https://raw.githubusercontent.com/d53dave/slack-black/master/black.css',
success: function(css) {
$("<style></style>").appendTo('head').html(css);
}
});
});