This;
I don't understand how Mint team can mess this up so badly. What gives?
Linux Mint uses the 16.04 base so themes should be compatible with GTK 3 in Ubuntu MATE 16.04. 17.04 uses a different GTK 3 version. It’s not the version of MATE which is important, but the version of GTK 3 being used as I understand it.
Right, so it’s GTK 3 screwing with MATE themes. Got it; Thanks GNOME! 
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It’s actually the Mint’s team doing this time 
Mint-Y is based on Arc, which does support multiple GTK versions, including the one in Mint and Ubuntu Mate 17.04. They seem to have stripped that support for some reason, maybe to ease the development.
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Ease development my ass. My bet is clem disliking people using Mint-Y in Ubuntu MATE so he is forcing users to be on his system with outdated software so Mint-Y renders correctly.
Stupid. There’s no reason for that kind of tomfoolery, especially if it will bite the dev team in the ass later when they have to upgrade MATE.