In application list, one is called themeing.
I just check, and the deactivation of theming failed
The theming app is always enable. It’s neither possible to install it separately nor possible to disable it. So I remain unclear what you did originally or are trying to do.
Trying to access array offset on false at /xxx/nextcloud/apps/settings/lib/SetupChecks/PhpOpcacheSetup.php#7
Please provide the full raw log entry / stack trace as described in the support template.
It would also be helpful if you provided your config, also as described in the support template.