As pointed out here #5626 (comment) the new icons are particularly ugly with the Tkagg backend. The reason is that Tk is using the ppm versions, which are not antialiased. One solution would be to use, e.g.,

convert -antialias -geometry 24x24 back.svg back.gif

etc. to generate antialiased gif files to replace the ppm files, and then change one line in backend_tkagg.py so that it will use these files.

In addition, the old and now unused xpm files can be deleted.