I think then you will see the special theme creator theme. But you can choose “default” in the sidebar theme toggle or your interface settings here at meta, or you create a new theme at theme creator and preview it without any changes.
@jidanni the change from “defer” to “mark unread” is is global change. You just have to wait for the next update of other Discourse forums.
https://github.com/discourse/discourse/pull/26468/files