There isn't anything, either in the FAQ or the guide that it links to, about editing mood themes. There is information about retrieving the mood theme ID for editing, but nothing about the actual process of editing.
So, something like the following:
It is possible to edit your mood theme later, using the same moodtheme_setpic command that you used to initially define the mood theme
(which is sucky wording, but someone can suggest a better wording)
needs to be added before the moodtheme_list information.
(Unless we want to rewrite the guide, either as a new guide or as part of the FAQ or as a howto tutorial thingie, in which case the rewriting would presumably include editing info, so the FAQ wouldn't need to, unless the rewrite gets jammed into the FAQ, in which case it would. Or something.)
And if this is green-thumbed into fruition, the title of the FAQ might be better off being something like "How do I create or edit a custom mood theme?" since those few people who actually try looking at the FAQs won't necessarily think to look for editing instructions under a creation FAQ.