/usr/share/sddm/themes/debian-elarun/theme.conf is in sddm-theme-debian-elarun 0.17.0-1ubuntu7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | [General]
background=/usr/share/desktop-base/active-theme/login/background.svg
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