/usr/share/themes/Radiance/index.theme is in light-themes 16.10+18.04.20180421.1-0ubuntu1.
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=Radiance
Comment=Ubuntu Radiance theme
Encoding=UTF-8
[X-GNOME-Metatheme]
GtkTheme=Radiance
MetacityTheme=Radiance
IconTheme=ubuntu-mono-light
CursorTheme=DMZ-White
ButtonLayout=close,minimize,maximize:
X-Ubuntu-UseOverlayScrollbars=true
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