There are no 3rd party watch faces, but there is now an ability for apps to ‘create‘ watch faces. It just uses the standard Apple Watch faces, but an app developer can build a face with complications from their app and make that watch face available within their app. It‘s nothing that you couldn’t build on your own using one of the standard faces and the app’s complications, but it can make it easy for third party developers to showcase their complications, I guess. It’s basically watch face sharing from within an app.