What is the theme in A Great Day by Frank Sargeson?

One major theme in the story is vengeance, as illustrated through Frank's belief that he has an issue with Ken. Frank has suffered obvious loss in the story, but his loss of Mary..... who has simply gone to work in Ken's aunt's home seems to have driven him over the edge. His best friend now works and isn't available, leaving Fred, who is jobless, without a companion. It is also alluded that possibly Ken has ambarked on some type of relationship with Mary, though, we are not told that directly.