What Theme Component is Discourse using for this?

[Moin]

It’s a custom component.

You can create a similar banner yourself.

Creating a banner to display at the top of your site

This guide explains different methods to create and customize banners on your Discourse site, from simple dismissible banners to more advanced options. Required user level: Moderator (Dismissible banner), Administrator Banners are an effective way to communicate important information to your site visitors. This guide will walk you through various methods to create and customize banners on your Discourse site. Welcome banner A welcome banner greets new and r…