When I change the theme, the KDE system settings window does not change.
For some reason, it is using a dark theme window with a light theme font - so I cannot read the settings. And when I change the theme, the System Settings window doesn’t change at all.
Thank you for your response! You put me on the right path with removing and reinstalling the themes and defaulting all their settings.
That ended up leading me to the /etc/environment file.
It turns out that there was an entry in there to set the QT_QPA_PLATFORMTHEME environment variable.
Once I removed that - everything went back to normal.