You can add custom JS or CSS very manually by adding a style tag just under the body tag in this file:
/var/www/control-panel/ui/index.html
I've edited mine like so:
<!doctype html><html lang="en"><head><meta charset="utf-8"/><meta name="viewport" content="width=device-width,initial-scale=1"/><meta name="theme-color" content="#000000"/><link rel="manifest" href="/manifest.json"/>
<title>Hosting Control Panel</title>
<script defer="defer" src="/static/js/main.7da93772.js"></script><link href="/static/css/main.dd28b799.css" rel="stylesheet">
<script>
// custom JS snippets here for chat widget, analytics, etc
</script>
</head>
<body>
<style>
.css-5yh0dl { display: none; } /* hide DNS alert box on site */
</style>
<noscript>You need to enable JavaScript to run this app.</noscript><div id="root"></div><div id="radix-root"></div><div id="mobile-portal-root"></div></body></html>
