How to use Cinnamon 3.8 theme stuff in Cinnamon 4.x
 

Running Mint 18.3 Cinnamon on one box, Mint 19.1 Cinnamon on another, and Manjaro Cinnamon on a third.
(Yes, I like Cinnamon.)
The issue for me here is part of the themes, specifically: how do I use the "Crux" Window Borders from Mint 18.3 in either of the others?
All 3 of them have the files installed in the File System, but it only shows up in "Settings" in Mint 18.3

So how can I get Settings > Themes > Window Borders to make "Crux-or-whatever" available?

Found the answer over on the Mint forums - I had missed it when I first looked.

Refer https://github.com/smurphos/Window_Borders_Mint_19/

Works with other Cinnamon distros, too, like Manjaro.