Welp, unfortunately, it does seem like the Core theme is borked for these forums. Maybe if more people took advantage of it then it would be the Default theme that was borked instead.
So, if you are unable to access the forums with your account then that is the most likely reason why. Currently, the forums show 61 users using the Core theme. For added fun, the forums won't show the names of those users so it will be a matter of trying to track them down and then getting them word on the steps needed to change their theme to default to access the forums again.
Currently, these are users who I can see have the Core theme still:
Insanolord
BlackNMild2k1
Mop it up
Stratos
BeautifulShy
Crimm
Arbok (last active 2019)
Athrun Zala (last active 2009)
Bill Aurion (last active 2012)
Bustin98 (last active 2020)
Ceric (last active 2017)
ClexYoshi (last active Feb. 2024)
Deguello (last active 2012)
jrlibrarian (last active 2014)
13/60
The hunt begins.