The main theme might be perseverance. Just like the title implies, the little sandwich 'could' which suggests it overcame something. It might be about the sandwich persevering through difficulties to reach its destination or purpose.
Well, I'm not super familiar with this specific 'the little sandwich that could story' but generally it might be about a little sandwich that overcomes some sort of obstacle or challenge. Maybe it's a story where the sandwich has to travel far and still stay fresh, or perhaps it has to be really delicious to win over someone's taste buds in a tough situation.
The sandwich is said to have originated from John Montagu, the 4th Earl of Sandwich. He was a gambler who didn't want to leave the gaming table to have a meal. So, he asked his servants to put meat between two slices of bread. This convenient way of eating soon became popular and was named after him.
A 'sandwich story' could refer to a narrative structure where a central event or idea is surrounded or sandwiched between other related elements or plotlines.
It could be a really wacky and creative story. Maybe Zephyr Breeze discovers a magical cheese sandwich that can grant wishes. And then she goes on an adventure to find more of these special sandwiches to help her friends in Ponyville. It might involve lots of humor as she tries to keep the sandwiches safe from villains who also want their power.