slack_theme.js

// After every Slack update, append to /usr/lib/slack/resources/app.asar.unpacked/src/static/ssb-interop.js:

// First make sure the wrapper app is loaded
document.addEventListener("DOMContentLoaded", function() {

// Then get its webviews
let webviews = document.querySelectorAll(".TeamView webview");

// Fetch our CSS in parallel ahead of time
const cssPath = 'https://cdn.rawgit.com/widget-/slack-black-theme/master/custom.css';
let cssPromise = fetch(cssPath).then(response => response.text());

let customCustomCSS = `
:root {
  /* Modify these to change your theme colors: */
  --primary: #A00000;
  --text: #ABB2BF;
  --background: #282C34;
  --background-elevated: rgba(40, 30, 30, .5);
}
div.c-message.c-message--light.c-message--hover {
  color: #fff !important;
  background-color: #222 !important;
}

div.c-message_attachment {
    color: #ABB2BF !important;
}

span.c-message__body,
a.c-message__sender_link,
span.c-message_attachment__media_trigger.c-message_attachment__media_trigger--caption,

div.p-message_pane__foreword__description span {
  color: #afafaf !important;
}

pre.special_formatting {
  background-color: #222 !important;
  color: #8f8f8f !important;
  border: solid;
  border-width: 1 px !important;
}
.c-message, .c-virtual_list__item {
  background-color: #222 !important;
}
`

// Insert a style tag into the wrapper view
cssPromise.then(css => {
  let s = document.createElement('style');
  s.type = 'text/css';
  s.innerHTML = css + customCustomCSS;
  document.head.appendChild(s);
});

// Wait for each webview to load
webviews.forEach(webview => {
  webview.addEventListener('ipc-message', message => {
      if (message.channel == 'didFinishLoading')
        // Finally add the CSS into the webview
        cssPromise.then(css => {
            let script = `
                  let s = document.createElement('style');
                  s.type = 'text/css';
                  s.id = 'slack-custom-css';
                  s.innerHTML = \`${css + customCustomCSS}\`;
                  document.head.appendChild(s);
                  `
            webview.executeJavaScript(script);
        })
  });
  });
});