One final update. I ran another test. I took a backup of my server, before doing the plex dance of the show, which still contained the incorrect theme.
I subsequently then 1) stopped plex, 2) deleted the contents of /var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Metadata, 3) restarted the server and 4) refreshed all my plex libraries.
Plex then downloaded the correct theme for the show.
Based on my testing, I guess as theme changes happen so infrequently that the metadata refresh code does not update it and it needs to be forced via a manual nuke of the metadata or a plex dance of the show. @drzoidberg33 adding you for awareness in case this scenario is of interest.
