New solution

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.

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Original solution

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.

• Flat Remix