/usr/share/themes/Materia-dark/index.theme is in materia-gtk-theme 20180321-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Type=X-GNOME-Metatheme
Name=Materia-dark
Comment=A Material Design theme
Encoding=UTF-8
[X-GNOME-Metatheme]
GtkTheme=Materia-dark
MetacityTheme=Materia-dark
IconTheme=Adwaita
CursorTheme=Adwaita
CursorSize=24
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