Thanks for this information, kanishou.

I wasn't having any trouble with the themes resetting on restarting the device, but I was using the WordCity theme, and had noticed that it would occasionally revert to default when I would access the Application Manager.

It actually made the switch with a subtle fade from the theme's orange colour to the default blue, which had me thinking it was intentional at first until I realized the WordCity theme didn't reappear afterward..

I'm not sure if that's a related issue, but I've applied your edit, and it hasn't happened since, so hopefully this is a fix..

One minor thing I did notice, which you may want to edit in your OP:

The actual file name to edit is:

/etc/event.d/hildon-control-panel-personalisation