I think, for a quick fix, you could simply launch this prog via a script and assign a truncated GTK theme file to an environmental variable that controls the appearance of buttons. But that is the quick, dirty and ugly way out. The sources are small. mygtk and png contain some relevant code. I think one of the the colors statement  in png.c might be the culprit, but I do not really know gtdk_pixbuf well.
With kind regards,
PS. And if you want to fix it, it would be nice to add alpha-channel (transparency) support for pngs. I like the size (under 100k stripped)!