The theme could be centered around mystery. The woman in the green dress might be an enigma herself. The story could be about other characters trying to figure out who she is, where she comes from, and what her intentions are. The green dress might add to the air of mystery, perhaps it has some special significance or a hidden story behind it. It could also be a story about social status. The green dress could indicate her social standing, and the story could show how she navigates through different social situations because of it.
One possible theme might be love. The green dress could be a symbol related to her love interest. For example, she might meet someone special while wearing the green dress, and the story could explore the development of their relationship and how it affects her. Another theme could be transformation. The woman may change in some way throughout the story, and the green dress could be a marker of that change, either literally or symbolically.