What Kamikaze said, basically. The word 'partly' denotes less than half, less than the majority for sure. And from such, if it's partly either, it means that the majority is the opposite, to one degree or another.
(Ex. If it's 'partly rainy', let's say, it isn't mainly firey 😉 - I don't care if that example made technical sense, it's supposed to get my point across! >_<)