WordNet (r) 3.0 (2006):

theme song
    n 1: a melody used to identify a performer or a dance band or
         radio/tv program [syn: signature, signature tune,
         theme song]
    2: a melody that recurs and comes to represent a musical play or
       movie

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "theme song":
   Brautlied, Christmas carol, Kunstlied, Liebeslied, Volkslied, alba,
   anthem, art song, aubade, ballad, ballade, ballata, barcarole,
   blues, blues song, boat song, bridal hymn, brindisi, calypso,
   canso, canticle, canzone, canzonet, canzonetta, carol, cavatina,
   chanson, chant, chantey, croon, croon song, dirge, ditty,
   drinking song, epithalamium, folk song, hymeneal, lay, lied, lilt,
   love song, love-lilt, matin, minstrel song, minstrelsy,
   national anthem, prothalamium, serena, serenade, serenata,
   signature, song, torch song, war song, wedding song