/usr/share/afterstep/desktop/icons/Toggle-Plasma 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 3 4 5 6 7 | <composite>
<solid width=64 height=64 color="#00000000"/>
<scale height="48" width="48">
<img src="tiles/Plasma"/>
</scale>
<img src="normal/Lightswitch" x="16" y="16"/>
</composite>
|