To accurately analyze the excerpt from H.H. Munro's story "Dusk" and identify which detail about the setting best conveys the mood, I would need the specific text you're referring to. However, I can help guide you through the elements that typically influence mood in a literary setting.
In "Dusk," the time of day is significant; twilight can evoke feelings of melancholy, reflection, or foreboding, depending on how it’s described. Key details such as the descriptions of light and shadow, the weather, and the positioning of elements within the environment can all help create an emotional atmosphere.
If you can provide the specific excerpt or some details from it, I can give a more tailored response regarding which detail is most effective in establishing the mood.