In some cultures, the thumbs-up and/or thumbs-down gestures are very offensive. In many, however, the are quite useful for representing "yes, good", and "no. bad", which is why I used them originally (in 2002!)
It would be nice to provide an option for swapping out the default (thumbs-up/down) Undo/Redo icons with curved-arrows (like used in most adult applications; see Gimp, Konqeuror, Inkscape, OpenOffice.org, Microsoft Office, etc.).
Note: This is DIFFERENT from the ability to theme Tux Paint. I think when themeability has been added, all themes should try to provide both variations of icons for undo/redo.
It could be an option like --alternate-undo-redo-icons or somesuch. :)