/usr/share/themes/Blackbird/index.theme is in blackbird-gtk-theme 0.4+20160704~ab4a30e-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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Name=Blackbird
Type=X-GNOME-Metatheme
Comment=Blackbird theme
GtkTheme=Blackbird
MetacityTheme=Blackbird
IconTheme=Adwaita
CursorTheme=Adwaita
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