Abandonment could be another theme. The fact that there is no one left gives a strong sense of being abandoned, and the novel might delve into the feelings of being left behind and the search for meaning in such a situation.
Survival is likely a theme. With no one else around, the protagonist has to find ways to survive on their own, whether it's finding food, shelter, or dealing with potential threats.
One possible theme could be survival. If someone is the last one left, they must have endured some kind of ordeal. Another theme might be loneliness, as being the sole survivor can be a very isolating experience.
One possible theme could be unity. Despite the differences symbolized by 'right' and 'left', the Christmas spirit might bring everyone together.
One possible theme could be reminiscence. The things left behind would likely trigger memories and a longing for the past.
One possible theme could be survival. Since Ashley Reeves was left for dead, her story of fighting to stay alive would be a major theme. Another theme might be betrayal, if she was left in that state by someone she knew. And also, resilience could be a theme as she must have had to be strong to endure whatever happened to her.
Growth and self - discovery are also likely themes. The summer can be a time of change, and in a 'one summer novel', the characters might be on a journey to find themselves. For example, they could be away from home for the summer and learn new things about their personalities, values, and dreams.
One possible theme could be identity. As the characters go about their day in Dublin, they might be grappling with their personal identities, influenced by the city's history and culture.
Forgiveness could be another theme. Maybe someone needs to be forgiven or has to learn to forgive in order to make the most of this one more chance. Also, self - discovery might be a theme as the characters could use this new opportunity to find out more about themselves and what they really want in life.
Love and loss for sure. It's a common theme when there's someone who got away. Regret is also likely, as the characters may constantly think about what could have been.
Another theme could be transformation. The shift from zero to one could symbolize a character's growth or change throughout the novel. It might be about a character starting from a state of emptiness or lack (zero) and evolving into something more complete or self - realized (one). There could also be themes related to identity, as the characters might be defined by whether they are more like the zero or the one in the context of the story's world.
It could be about the struggle between good and evil. If there is a 'great unclean one', it might represent evil, and the story could revolve around characters trying to defeat or resist it.