For example, use a customized theme named AAA in Configuration X.
Then use [Reset All] command in Display tab in Configuration Properties dialog.
All the default themes are reset. The customized themes can not be found/selected in Configuration Properties dialog.
Then switch to Configuration X and open Configuration Properties dialog again.
All the settings of theme AAA are kept, though the theme has no name now. This is expectable.
*But the Save/Copy/Rename/Delete/Export commands can not be used on this theme.
I think this is a bug. These commands should be available in such cases.