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.
I'm not sure specifically which 'back in pampers story' you are referring to. There could be many personal or fictional stories with that title. It might be about someone's memories from when they were a baby in diapers (pampers).
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.
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. The letter might be a love letter expressing deep affection and passion.
One possible theme could be identity. If Parker has some sort of mark or feature on his back, it might be related to how he sees himself or how others perceive him.
One possible theme could be adventure. Just like the journey 'there and back again', it might involve exciting experiences related to water, like exploring uncharted waterways.
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.