The soundtrack as you walk into St. Luke’s ought to provide a hint: it’s American songbook standards, like “In Other Words” and “Fever,” rendered by the likes of Frank Sinatra and Peggy Lee, respectively. It primes the audience to expect a retro evening, and Love Quirks, the new musical by Seth Bisen-Hersh (music and lyrics) and Mark Childers (book), while set in the present-day New York of Instagram and Grindr and Twitter, is retro. It wants to be a sweet old-fashioned evening of melody and humor and light romance, and some of the time it succeeds.