I rebooted which is equivalent. The problem seems intermittent, yesterday I trashed the cache folder, restarted, re-selected a theme, restarted and it was ok. I restarted another time just to be sure that the problem didn't occur again. And this morning when my iMac booted, the problem re-occured. If I reboot, it doesn't go away.

I've seen no crash report in either /Library/Logs or ~/Library/logs.

Raf