Created by user https://gist.github.com/focus97 :)
Add this code (below) to the bottom of: Contents/Resources/app.asar.unpacked/src/static/ssb-interop.js
document.addEventListener('DOMContentLoaded', function() {
let tt__customCss = `
body, .channel_header, #footer, .channel_title_info, #channel_topic_text { background: rgb(0, 43, 54); }
.c-message__body { color: rgb(153, 174, 177); }
#team_menu, .p-channel_sidebar { background: #023f4e !important; }
.c-presence--active {color: rgb(177, 202, 17) !important;}
nav.p-channel_sidebar .p-channel_sidebar__channel--selected, .p-channel_sidebar__link--selected, .c-message_list__day_divider__label__pill, .p-message_pane .c-message_list.c-virtual_list--scrollbar > .c-scrollbar__hider:before { color: #eee !important; background: rgb(27, 139, 210) !important; }
.c-message_list__day_divider__line { border-top-color: rgb(27, 139, 210) !important}
#msg_input, #primary_file_button { background: rgb(2, 55, 68) !important; }
#msg_form #msg_input { border-color: transparent; }
`;
$.ajax({
url: 'https://cdn.rawgit.com/laCour/slack-night-mode/master/css/raw/black.css',
success: function(css) {
$('<style></style>').appendTo('head').html(css + tt__customCss);
}
});
});