Christina, a pretty and feminine, crystal clear classic, may be trending downward, but it's never out of style. Christina's short forms Chris, Christie, and Tina all seem dated—making the royal Christina best used in its full glory.
Christina became a royal name in Sweden (Greta Garbo played Queen Christina in one of her classic films) and Spain, and it is more usable today than the French Christine.
Christina is now well represented by actresses Applegate, Ricci, and Mad Men's Hendricks, alongside singer Aguilera and Orphan Train novelist Baker Kline. Previous bearers include British poet Christina Rossetti and Australian novelist Christina Stead.
Christiana is a more distinctive variation.