The theme of the story for me even though there many other themes out there it depends on how the reader see it for me the theme would be,
It is nice to value the things you have because one day you may not have them.
Evidence is that the story says that he kept all the letters in his sweater drawer and well kept, so like that he would know if someone disturbed the letters he loved so much.
Also another evidence that is in that paragraph is that the character only read the part he liked because it would make him happy and that means that he is valuing the letters a lot.