We released Center Row and The Daily in 2017, as well as Cozy for Omeka S. We’re currently seeking feedback on a new theme for Omeka S, as well.
Our theme repository is open to submissions, just like the plugins/modules directory. We’re always happy for people to share their themes but, as you say, these custom designs often don’t get generalized by their creators, or if they do they aren’t submitted.
Perhaps, if you have design students at your institution, they might be interested in building or adapting themes. It would be a good learning experience, especially working with your students as ‘clients’.