Hi Rob, pretty much as Neon puts it, having an agreed palette of colours and an agreed way of depicting buildings - e.g. a 3 point black stroke with a 50% gray interior....that kind of thing, it's really so that all the elements in the map are consistent - not sure if style sheet is the right word!
If we do have one, it would have to be a style that is reproduceable across as many applications as possible, e.g. CC, PS, Gimp, Illustrator etc.