/usr/share/messageviewer/about/default/splash.theme is in kf5-messagelib-data 4:17.12.3-0ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | [Desktop Entry]
Name=Default
Description=Default splash theme
FileName=main.html
|