upgrade to dark slack.

extension.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://raw.githubusercontent.com/Nockiro/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: #61AFEF;
     --text: #ABB2BF;
     --background: #282C34;
     --background-elevated: #3B4048;
  } 

  a[aria-label^="NAME_OF_CHANNEL_OR_DIRECT_CONVO_TO_STYLE"]
  {
       --background: #4d0000  !important;
       --text-transform: uppercase  !important;
       --letter-spacing: 2px !important;
       --text-shadow: 1px 1px white;

   }   `

  // 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);
           })
     });
  });
});

install.sh

# install:
# curl https://gist.githubusercontent.com/ssoroka/2e9bc1f1a0ab876cf201a124333cdbe8/raw/install.sh | sudo sh

set -ex

cd /Applications/Slack.app/Contents/Resources/app.asar.unpacked/src/static

curl https://gist.githubusercontent.com/ssoroka/2e9bc1f1a0ab876cf201a124333cdbe8/raw/extension.js > extension.js
cat extension.js >> index.js
cat extension.js >> ssb-interop.js
echo Done! Please restart slack.

see https://github.com/Nockiro/slack-black-theme