There are several possible themes. For instance, the power of kindness. Thumbelina was often kind to those she met, and the girl in the wishgarden might also display this trait and see how it affects her relationships. Another theme could be the search for home or belonging. Thumbelina was always looking for a place to belong, and the girl in the wishgarden might be on a similar quest. Also, the theme of magic and wonder is surely present as it is set in a magical wishgarden and likely has elements of the fantastical like the Thumbelina story.
Themes in this story could include courage. Thumbelina showed a lot of courage in her adventures, and the girl in the wishgarden might face her own fears. Also, friendship is likely a theme. In the original Thumbelina story, she made friends along the way, and the girl in this new story could form bonds with the creatures in the garden. Additionally, the theme of discovery, both of the self and of the magical world around her, could be prominent.