Any stage designer can put his or her imprint on a show that no one's seen before. But what do you do when you've got to dress up the longest-running musical on or off Broadway? That would be The Fantasticks. The Hollywood Playhouse staged the chestnut as the debut performance under its new management, and nobody who's seen the off-Broadway version could recognize the set. A boy, a girl, and a wall were the basic elements, as they have been for 40 years, but in place of a bare stage and black-box aesthetics, designer David K. Sherman substituted cotton-candy pastels and whimsical costumes. Think the show can't work with an entire palette of cheerful light filters that changed the performance area from pink to blue to midday yellow? Think again. As this production demonstrated, just because a show is timeless doesn't mean it has to be stale.