theme song

Assuming you mean the music where Gillian hums to Pyewacket, according to the playlist on the soundtrack LP, the track is called "The Spell"; The music played through the opening credits is listed simply as "Main Title". The composer/conductor is George Dunning, who also created the music for "From Here To Eternity", along with "The Eddie Duchin Story", & the hauntingly beautiful score from "Picnic", both of which starred Kim Novak.
The soundtrack was released by Colpix (Columbia's in-house label) in 1958 in conjunction with the movie, as COLPIX #CP 502. It was released again in 1976 on the Citadel label - CITADEL #CT006. (LP only, as near as I've ever been able to discover.) The liner notes about Dunning & the Brothers Candoli, who performed the jazz numbers at the "Zodiac" nightclub, are excellent, and fascinating in their detail. For instance, the piano player on the jazz sessions is none other than John Williams, very early in his career! Both versions of the LP turn up on e-bay from time to time...