I don't think Radioactive has the right feel for Obernewtyn if that makes any sense?? It certainly relates to a lot of the contexts, but the song feels more suited to a steampunk/sci-fi kinda post apocalyptic society while Obernewtyn has always had more basis in fantasy when it comes to the general feel of the world. (Though I do really like that song)
A lot of stuff by Florence and the Machine makes me think of Ober, not due to lyrics, but due to the feel of some of the music because it sometimes has that kinda mystical, powerful edge to it. Also maybe Blackmore's Night--but they are more folky (
Under a Violet Moon is something I could see people singing at moonfair and they have a song called Ariel where the lyrics could actually be quite relevant).
But yeah, I have no clue for theme songs.