What is the theme of Fern Hill by Dylan Thomas?

I always have trouble understanding themes and have trouble finding them. I know it has to something with childhood and time, but what does it say?

What is the theme of Fern Hill by Dylan Thomas?
I have only just read this, and so haven't been able to absorb it all....my immediate reaction is to say it is a poem about nostalgia, a longing for a time long past, a longing for a more natural world that follows the rhythms of nature and the farming community. The theme could just be called Remembering, as that is what he is doing. He is an adult remembering his childhood and youth, a time less complicated than the present.