Okay, here's the full 20-theme batch I've found, including the default, for anybody willing to go for the shotgun approach.
I'll do individual posts for each one after I get some screen caps.
BTW, when I tried copying them over en-masse to the System\Themes folder, they didn't all show up in the Settings menu; I suspect the menu size is limited.
Edit: On the other hand, I tried it again and I noticed that with 15.1 the listing scrolls so they all show up. Maybe I just missed that in 14.2.