/usr/share/themes/Greybird/index.theme is in greybird-gtk-theme 3.22.8-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 | [Desktop Entry]
Type=X-GNOME-Metatheme
Name=Greybird
Comment=Greybird theme
Encoding=UTF-8
[X-GNOME-Metatheme]
GtkTheme=Greybird
MetacityTheme=Greybird
IconTheme=Adwaita
CursorTheme=Adwaita
ButtonLayout=:minimize,maximize,close
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