toappendSlack.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://gist.githubusercontent.com/Gungthar/30716df122a6919f4377f91f828b2652/raw';
  let cssPromise = fetch(cssPath).then(response => response.text());

  let customCustomCSS = `
  :root {
     /* Modify these to change your theme colors: */
     --primary: #8EF03B;
     --text: #CCC;
     --background: #080808;
     --background-elevated: #222;
  }
  `

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