You can achieve basically the same thing as the theme by adding CSS like this to the end of ~/.config/gtk-3.0/gtk.css:
decoration {
box-shadow: black 0 0 0 3px, #eeeeee 0 0 0 1px;
}
This seems way cleaner than the original solution, and probably works with all themes. I only checked a few.
This is a Flat Remix theme adjusted to have white borders around windows for better contrast. NOTE: Looks like Window Borders were removed from Theme settings, so this is useless on new versions of Cinnamon.