One possible theme could be survival. If darkness represents a great danger, then the characters in the stories would be focused on surviving it. Another theme might be hope. Despite the impending darkness, there could be characters who still hold onto hope and strive to make things better.
I haven't read this work in detail. But generally, when analyzing such a collection, one could start from the themes. It might deal with the concept of darkness, perhaps in a metaphorical sense like the fall of morality or the approach of some kind of doom. The related stories could expand on this central idea, maybe by showing different characters' reactions to the threat of 'darkness'.
Well, it could be centered around the idea of hope in the face of impending doom which is symbolized by the 'darkness'. Maybe the characters in the novel are fighting to keep their world from being consumed by this 'darkness', whether it's a physical, moral or spiritual darkness. They might be trying to uphold values like light, goodness and order.
I'm not sure specifically as I haven't read it, but it could be a hero figure who is trying to prevent the darkness from falling. There might also be a villain or some sort of antagonist that represents the forces of this 'darkness'.
Probably one of the main themes is the struggle between good and evil. As the title implies, the characters might be falling into darkness, which means they are likely being tempted by evil forces or their own dark desires.
One possible theme could be scandal. Given Wallis Simpson's controversial relationship with Edward VIII, scandal is likely to be a major theme.
Another theme could be the journey of self - discovery. The house of darkness might be a place of confusion and self - doubt, while the house of light is where the character finds their true self. It's like a metaphor for the internal battle within a person.
One possible theme could be the struggle against the unknown. Darkness often represents the unknown in literature, and characters may have to fight their fears.
One possible theme could be courage. Riding in darkness can be scary, so the story might be about someone finding the courage to do it.
One possible theme could be isolation. The voice in the darkness might make the character feel more alone, even in a world full of other beings like in Twilight.
One possible theme is the struggle against the unknown. The darkness in the labyrinth represents the unknown, and the characters have to face their fears to progress.