The theme of The Hunchback of Notre Dame, by Victor Hugo, is difficult to narrow down to a single one, as there are many that intertwine throughout the narrative. Perhaps the greatest consistent theme throughout, however, is that of love, especially sacrificial love, and especially when everything else—social standards, cultural expectations, others' opinions, even perhaps your own prejudices and hesitations—seems to stand against love as completely irrational and/or ridiculous.
One of the clearest examples in Hunchback of an obstacle that some would consider insurmountable to true love is the physical appearance of the titular character, however shallow or unkind an obstacle that may seem. Quasimodo is consistently described as unsightly to behold. He therefore assumes himself to be unloveable, and certainly the townspeople and even Frollo consider him to be unloveable, as though Quasimodo's physical appearance is the only trait that matters.
And yet, throughout the novel, there are numerous instances of love, with varying degrees of genuineness and success: Esmeralda's love for Phoebus, Quasimodo's love for Esmeralda, Frollo's so-called "love" for Esmeralda, Quasimodo's "love" (of sorts) for Frollo, and so on. Indeed, perhaps the theme of the novel can be best summarized in a line that Victor Hugo gave to the story's narrator: "Love is like a tree: it grows by itself, roots itself deeply in our being and continues to flourish over a heart in ruin. The inexplicable fact is that the blinder it is, the more tenacious it is. It is never stronger than when it is completely unreasonable" (book IX, chapter IV).