/usr/share/themes/Ambiance/index.theme is in light-themes 0.1.9.1-0ubuntu1.
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=Ambiance
Comment=Ubuntu Ambiance theme
Encoding=UTF-8
[X-GNOME-Metatheme]
GtkTheme=Ambiance
MetacityTheme=Ambiance
IconTheme=ubuntu-mono-dark
CursorTheme=DMZ-White
ButtonLayout=close,minimize,maximize:
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