So, it seems like Situations app just reads ambience list from /usr/shate/ambience. Usually that only contains built-in ambiences, but if you use Theme Colors app by @nephros to create your ambience, it installs there, so your custom ambience will show up on the list.
I’m not sure where the custom ambience files are kept, (nephros probably knows) - I guess a typical custom “ambience” is just a picture, so there’s probably no such directory…