I don't know if this is the same issue, but it looks to me like there are two icons it doesn't pull from the GTK theme at all. The offending xarchiver toolbar buttons are labeled "Add files" and "Extract files," and are just labeled "Add" and "Extract" under the "Action" menu.
The icon theme I'm using, under ...128x128/actions/, has the following files and symlinks:
I don't know if this is the same issue, but it looks to me like there are two icons it doesn't pull from the GTK theme at all. The offending xarchiver toolbar buttons are labeled "Add files" and "Extract files," and are just labeled "Add" and "Extract" under the "Action" menu.
The icon theme I'm using, under ...128x128/actions/, has the following files and symlinks:
add-files-to-archive.png --> gtk-new.png
gtk-add.png --> add.png
add-folder-to-archive.png
extract-archive.png
I suspect the first and and last of those should be detected and used.