xcursor-transparent-theme doesn't provide index.theme by default, this might cause 'gtk-update-icon-cache -q /usr/share/icons/xcursor-transparent' failed on target: Error: gtk-update-icon-cache: No theme index file
The conclusion was that postinst should check if there is index.theme or ignore it missing, not adding index.theme. The gtk-icon-cache class uses -t when updating the caches: --ignore-theme-index, -t Don't check for the existence of 'index.theme' in the icon theme directory. Without this option, gtk-update-icon-cache refuses to create an icon cache in a directory which does not appear to be the toplevel directory of an icon theme. If someone is not using gtk-icon-cache.bbclass they either should, or have a good reason and also use -t.
Setting to VERIFIED as per above comment. Please open a new bug if necessary.