This is a bookmarking site running on software called Postmarks. It’s currently running in “standalone” mode, meaning it doesn't broadcast any activity to the rest of the internet via the ActivityPub protocol. You can still subscribe to new bookmarks using the Atom feed, which any modern RSS/feed reader should support.

Postmarks (in its default configuration) uses an edited version of Eynav Raphael’s “Postmark Stamp” icon from The Noun Project. It also makes use of free fonts including Averia Sans and Public Sans.