Nostalgia could be a theme. Since it's about a place the person used to live, they might be feeling nostalgic for the past.
It could be about someone's past experiences in a particular place. Maybe it's a nostalgic look back at a time when the narrator had a connection to that location.
One possible theme could be identity. Maybe the characters are trying to figure out who they are in the context of the story's setting.
It could be about love. Love is a very common theme in short stories, like a short-lived romance or unrequited love.
A major theme is motherhood. The mother is constantly grappling with her feelings of inadequacy in bringing up her daughter.
I'm not sure specifically as there could be many short stories with such a title. It might be about something that was once in a natural or green state, perhaps a place like a forest that has changed over time, or an object that was green before but has lost its color or its original nature.
Another theme could be change and transition. The death of the king is a significant change, and the ascension of the new king represents a new beginning. It might explore how the kingdom and its people adapt to this change. Also, it could touch on themes of loyalty, as the subjects now have to transfer their loyalty from the old king to the new one.
One possible theme could be isolation. If nobody knows someone is there, they are likely cut off from others in some way.
I'm afraid I can't tell you the author of the 'it used to be green once' short story. It could be an indie author or a relatively unknown writer. Maybe it was written by someone for a local publication or a small - scale writing competition.
Love and loss could be a major theme. Since it's about a woman once loved, there's likely to be a sense of what was lost in that relationship.
One possible theme could be fear. The dark often represents the unknown, and in a 'dark once upon a time story', characters usually have to face their fears of the dark and what it might hold.