I've been playing with the custom mood theme editor over the last few days. for a request, (which closed without credit, so there's no point linking to it) and for a very unofficial cheet sheet tutorial I wrote for a community. Using the Custom mood theme editor raises an idiosyncrasy that doesn't seem to be recorded in the FAQ, and is a bit confusing. When you are using the "inherit from" check boxes to set lower level moods to display the higher level mood theme pictures, and you click the check boxes, leave the check marks in the check boxes, and hit "save changes" nothing happens. No changes are saved.
For the custom mood theme editor to save your changes, you need to check the "inherit from" check boxes, allow the URLs and mood theme pictures to appear, (the URLs appear uneditable and in red) and then uncheck each box so that the URLs turn black, before saving the changes. You can also paste the URLs of your mood pics in here directly, or just type in the name of the image file in the URL that's there already.