Themes such as perseverance and courage could be in the story. The girl needs to persevere through the long periods of separation. She has to be courageous in facing the possibility of losing the soldier in combat. And there might also be a theme of patriotism, as the soldier is serving his country and the girl may support him in that noble cause.
Love could be a theme. The love between the soldier and the girl, and how they maintain their relationship despite the challenges of military life.
Another theme could be courage. The girl might show courage in waiting for the soldier and facing the uncertainties of his military life.
One possible theme is sacrifice. The soldier may have to sacrifice his time with his loved one for his duty. Another theme could be loyalty, both to his country and to his partner.
One possible theme is sacrifice. Soldiers sacrifice their time with family during Christmas for the greater good.
Love and protection are common themes. A father's love for his daughter often drives him to protect her from harm.
Friendship could be a theme. Maybe the story is about how someone formed a strong bond with the girl upstairs, through shared interests or difficult times.
One possible theme could be survival. The girl in the basement would have to fight to stay alive, whether it's against the elements, lack of food, or an abductor.
One possible theme could be isolation. Being in the closet implies being cut off from the outside world.
One possible theme could be self - discovery. The girl might go through experiences on the mountain that make her learn more about herself.
One possible theme could be survival. If she was in the bunker, she must have been trying to survive whatever situation led her there.