Yeah, not everything works out of the box yet. To get it working, you need to first install the gnome-shell-extensions package, as that contains the user-theme extension.
Also if you are on Update Pack 4, the version of gnome-tweak-tool has a few known bugs and it won't detect the user-theme extension due to it having a different UUID to what it is looking for. What you would need to do is to temporarily enable the Debian Testing repository and upgrade only the gnome-tweak-tool package, then disable Testing again afterwards. Version 3.2.2-3 should work properly.
LMDE (tracking Testing) with Cinnamon; Intel Core 2 Duo E8400, Nvidia GeForce GTX 260 (using Nouveau)