The source for all the icons is dox.svg

The shell script buildicons.sh is run (on Linux, most likely) to
generate a windows icon in dox.ico and a set of PNGs of assorted
sizes (which will be converted to a .iconset and .icns
for adding to the bundle using iconutil at compile time on darwin).

The source SVG is under a CC3.0 license:
<div>Icons made by <a href="https://www.flaticon.com/authors/freepik"
title="Freepik">Freepik</a> from <a href="https://www.flaticon.com/"
title="Flaticon">www.flaticon.com</a> is licensed by
<a href="http://creativecommons.org/licenses/by/3.0/"
title="Creative Commons BY 3.0" target="_blank">CC 3.0 BY</a></div>