User blog comment:BertH/Introducing Announcements, a new way to notify your community/@comment-24155515-20180717201545/@comment-5275700-20180717211644

Announcements is a service-based part of the platform, like Discussions, and so the MediaWiki-based styling is not applied there. And since the management page is not a Special page, it's not listed on the "Advanced" tab of Admin Dashboard (which is really just Special:SpecialPages, as you probably know.)

Help:Community messages is up-to-date documentation for that tool.