Determining themes for previous Tanglewood opening night galas always came swiftly and easily for Nina Jung, the Boston Symphony Orchestra’s in-house event designer. Four years ago when the season opened with Hector Berlioz’s epic Trojan War opera “Les Troyens,” she whipped up a themed menu and party — togas optional.

This year, as the BSO celebrates its 75th season in the Berkshires, Jung was faced with a challenge: There is no clear musical theme for opening night.