Suggest that something about this be added to FAQ 64.
Instructions on how to create a custom mood theme can be found at http://www.livejournal.com/doc/find/?guide=moodicons. You can view a particular mood theme including the mood theme IDs by appending &mode=tree to the mood theme URL: http://www.livejournal.com/moodlist.bml?moodtheme=<moodthemeID>&mode=tree. Note that once a custom mood theme is created, it cannot be deleted.