I can confirm that gtk3-engines-breeze 5.6.95-1 works with GTK 3.20 but not with GTK 3.18 (as used in Ubuntu 16.04 LTS).
On the other hand, I don't see that any of the other GTK 3 themes in Debian depend (or break) on GTK 3 at all. It's possible for a GTK 3 theme to support multiple versions by using directories like /usr/share/themes/gtk-3.20/ , gtk-3.18/ , gtk-3.0/ , etc. That's not very important for Debian which doesn't support multiple GTK3 versions in the same Debian release but it could be useful for a theme developer. Thanks, Jeremy Bicha
