/usr/share/themes/Numix/index.theme is in numix-gtk-theme 2.1.2-0ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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Type=X-GNOME-Metatheme
Name=Numix
Comment=A modern flat theme
Encoding=UTF-8
[X-GNOME-Metatheme]
GtkTheme=Numix
MetacityTheme=Numix
ButtonLayout=:minimize,maximize,close
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