what is the theme of the ch-on the face of it?

The theme of the story is that actual strength and beauty comes from within. Derry's disfigured face and Mr. Lamb's tin leg might make them appear less beautiful or not pleasant to the eyes of others but that has nothing to do with their inner beauty as human beings. Beauty is not in the eyes of the beholder but in the positive attitude and inner strength of those who possess it.