Which detail most strongly develops the theme of loneliness in "The Raven"?

The second stanza is remarkably effective at highlighting the speaker's loneliness and sense of desolation. Recalling that strange night when the raven visited him, he refers to the "bleak December" and how each "dying ember wrought its ghost upon the floor." These details provide a suitably gloomy backdrop to the events of the poem as they unfurl.
The speaker is all alone in his study, brooding over the death of his beloved Lenore, eagerly wishing that tomorrow will come. As well as being alone physically, he is also alone with his thoughts of his lost lady love—and will be throughout the rest of the poem.
At first, it seems that the raven will provide some distraction for the speaker: will take his mind off Lenore. But with each insistent squawk of "Nevermore," the speaker becomes ever more deeply mired in loneliness and sorrow, until eventually he screams at the bird to leave his loneliness unbroken.
https://www.poetryfoundation.org/poems/48860/the-raven