Final answer:
Leonard Bernstein composed the music for the 1957 musical West Side Story, contributing to its long-standing success despite its darker themes.
Step-by-step explanation:
The composer who created the music for West Side Story is Leonard Bernstein. The musical, which debuted in 1957, was a collaborative effort involving several key figures: Jerome Robbins as the original director and choreographer, Arthur Laurents providing the book, and Stephen Sondheim joining later as the lyricist. Bernstein's score for West Side Story is renowned for its blending of different musical styles and for pushing the boundaries of musical theater at the time. Despite initial skepticism about the dark and tragic themes in the show, Bernstein's music, alongside the work of his collaborators, contributed to the musical's profound impact and enduring popularity.