How would I present the themes of "The Listeners" by Walter de la Mare using SlideShare?

It sounds like you have been asked to present the themes of Walter de la Mare's evocative poem "The Listeners" in the form of a slideshow or presentation. As such, I will try to break them down for you in an appropriate format.
1. "The one man left awake." While the title directs us to view the Listeners, and not the Traveler, as the subject and core focus of the poem, there is certainly a theme surrounding the particular kind of isolation that comes from being the sole survivor of something, the only one left to remember it. "The Listeners" was a late poem for de la Mare; he wrote it in the latter years of his life, after his wife had died. While it is dangerous to read things into poetry based upon the author's biography, it could certainly be suggested that people late in life often do feel this sense of being "the lone voice from the world of men," the "lonely Traveler" apparently speaking only to "phantom listeners." On his journey, the Traveler has bound himself to an agreement, which was important to him, but now, he is the only one who remembers—"Tell them I came, and no-one answered." This worldly bond is now only important to the Traveler; the phantom Listeners no longer care to keep whatever arrangement was made.
We also see this theme of "the sole survivor" in poetry of the Second World War; this poem was written in 1947, when the memory of that conflict was very recent.
2. We might identify silence as a second theme of the poem. The poet goes to great lengths to emphasize the quietude of the forest, which is broken only by the Traveler's voice. In the "silence," in the "empty hall," and in the "stillness" of the phantom listeners, the poet conveys the all-pervasive isolation and emptiness of the house, which is yet "thronging" with something unidentifiable. Silence, and the breaking of silence by "the lonely Traveler's call," has become its own almost animate thing, capable of "answering" the Traveler; it is all that is left of what came before.
3. "Strangeness," or the unknown. This is a theme that ties in strongly to the poet's mood and tone. This poem by de la Mare does focus more on atmosphere and mood than on theme, specifically. But we never do discover who the Listeners are. The Traveler is aware that they are there; it is unclear to us, though, whether he knows something we don't when he feels "their strangeness, their stillness answering his cry." He is "perplexed and still," and yet he continues to speak to this "host" of people from another world whose presence is yet palpable to him in the "dark house." The poem closes without ever giving explanation, leaving us eerily unsure as to what bargain the Traveler once made, who is listening to him, and where he is now bound on his journey.