What is the theme of the poem "To a Skylark" by Percy Bysshe Shelley?

The theme of Shelley's poem "To a Skylark" is the power  of nature to transform men's lives, specifically through the medium of poetry.

The skylark is a tiny bird, so small that when it flies high in the heavens it cannot even be seen by the author, yet its song can still be heard, a song "unbodied joy" (line 15) and "shrill delight" (line 20).  The author hears the skylark and goes on to describe its beauteous song, but it is "a flood of rapture so divine" (line 85) that he cannot fully capture its essence.  The joy expressed by the skylark is beyond that which can be grasped by man, and the author speaks directly to the skylark in the latter stanzas, asking it to reveal to him the secret of its ethereal bliss so that he might then be able to share it with others through his words, and thus transform their lives.