/usr/share/afterstep/backgrounds/Smooth.mini is in afterstep-data 2.2.12-6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | <gradient width="$minipixmap.width" height="$minipixmap.height"
colors="BaseDark BaseLight" angle="45"/>
|