On Monday, the actress Sydney Sweeney, who is no stranger to controversy, posted a video on Instagram depicting her with a group of people clad in all black scaling the Hollywood sign under the cover of night. The crew were carrying multiple duffel bags jam-packed with bras that they draped over the landmark. As their mission wrapped, Ms. Sweeney stood in front of the sign, proudly beaming as she promoted her new lingerie brand, Syrn (pronounced “siren”), which launched Wednesday.