Ah that makes sense.
I definitely do have my 0.7 and 0.8 unzipped into separate folders. I just deleted both and re-unzipped from scratch, and I notice all these extra themes are no longer in my 0.8 folder.
I must have accidentally combined them at some point, and just don't remember.
Anyway, if the theme format is so incompatible between 0.7 and 0.8, it would probably be a good idea to include some sort of version check in the theme .json files. It could be as simple as adding something like:
"Theme Version":0.8,
... at the top of the newer theme JSON files. That way if the selected theme is a lower version than the current Masterplan version compatibility, it would just display some message and not load the broken theme.