User blog comment:MisterWoodhouse/The Unified Community Platform Editor and our first release date/@comment-39541499-20200310071025/@comment-38078079-20200311095524

I'm pretty sure that user is right, though.

I could stand corrected, but I believe only new wikis - at the start - will be on UCP. Your wiki will switch over to UCP in time, but the UCP is being introduced in stages, so there's no current estimate when that would be. I doubt there'd thus be a JS allowed to import the design.