/etc/xdg/messageviewer_header_themes.knsrc is in kf5-messagelib-data 4:16.04.3-3~deb9u1.
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 | [KNewStuff3]
ProvidersUrl=http://download.kde.org/ocs/providers.xml
Categories=KMail Header Theme
TargetDir=messageviewer/themes
Uncompress=always
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