Modify the file ssb-interop.js file to include the following at the end of the file:

document.addEventListener('DOMContentLoaded', function() {
 $.ajax({
   url: 'https://cdn.rawgit.com/laCour/slack-night-mode/master/css/raw/black.css',
   success: function(css) {
     let overrides = `
     code { background-color: #535353; color: #85c5ff; } /* Change color: to whatever font color you want */
     .c-mrkdwn__pre, .c-mrkdwn__quote { background: #535353 !important; background-color: #535353 !important; }
     `
     $("<style></style>").appendTo('head').html(css + overrides);
   }
 });
});

In Arch Linux this file is located at : /usr/lib/slack/resources/app.asar.unpacked/src/static/ssb-interop.js
With the mlocate package installed simply find the file on your system using : sudo updatedb && locate ssb-interop.js

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