Another theme could be loss. Maybe the letter was written in the wake of losing someone or something important, like a letter written by a person mourning the death of a loved one, sharing memories and the pain of the loss.
One possible theme could be love. The letter might be a love letter expressing deep affection and passion.
A third theme could be adventure. For instance, if it's a letter from an explorer, it would be filled with tales of new places, strange encounters, and the excitement of discovery.
One possible theme could be isolation. Silence often implies being alone or cut off, and in a back story, it might show a character's struggle with feeling isolated from society or their loved ones.
Nostalgia could be a theme. If it's a story looking back at the time when someone was in pampers, it's often about reminiscing and feeling nostalgic for that period of life. Also, growth and development could be themes. The story might show how a person has changed from being a baby in pampers to who they are now.
Another theme could be the loss of innocence. The war would have exposed people to violence and death, and those involved might have lost their naivete. For example, young soldiers going to Vietnam and coming back changed forever. Also, a theme of friendship might be present. Soldiers or civilians might form strong bonds during those difficult times to support each other.
One possible theme could be regression. It might explore the idea of going back to a more innocent or dependent state, like infancy when diapers are used.
One possible theme could be love and longing. The wife might be expressing her deep love for her husband and how much she misses him in the letter.
One possible theme could be deception. If the letter contains false information or is used to mislead someone, that would be a major theme.
Another theme might be change and adaptation. If it's a story about someone who has to go back to diapers due to an unforeseen circumstance, they would have to adapt to this new situation, whether it's a physical or mental adjustment.
A third theme could be hope. Despite all the hardships in 2020, if it's a real story about 'the way back', there must have been some element of hope that drove the protagonist forward. It could be hope for a better future, hope to reunite with loved ones, or hope to regain something that was lost during that eventful year.
One possible theme could be regression. It might show a character going back to a more infantile state by having to wear nappies again.
One possible theme could be acceptance. The story might be about how the 'fat lad at the back' is eventually accepted by his peers or the group he is in.