Il CSS va aggiunto al tuo sito, non al CSS incorporato di Discourse. Quindi dovresti avere qualcosa di simile a questo:
<html>
<head>
<script src="https://jonathan5-discourse.com/javascripts/embed-topics.js">
</script>
<style>
d-topics-list iframe{
width: 100%!important;
}
</style>
</head>
<body>
<d-topics-list discourse-url="https://jonathan5-discourse.com" category="5" per-page="5"></d-topics-list>
</body>
</html>