Layered Themes

The ability to be more specific with the themes we want the AI to select from / use by layering themes, such as Castle theme and Ruins theme to get elements appropriate to a ruined castle. This is likely to become more and more useful as more themes are incorporated into the tool, but some are fairly obvious - such as layering sub themes such as "Ancient", "Arcane", "Religious", "Ruined", "New", etc