Can you dig it? The Fosters celebrates Lena’s (Sherri Saum) big 4-0 with a trip to the ’70s on the July 25 episode of the Freeform drama, as the family’s backyard is transformed into a Studio 54–inspired club for the party. The whole cast (Maia Mitchell, Tom Williamson and David Lambert, top; Teri Polo and Saum, bottom left) got into the theme. “Our costume designer is so talented and meticulous,” says executive producer Joanna Johnson. “She put together some fantastic iconic ’70s looks.” Of course, no family occasion comes without drama, this time courtesy of Lena’s mom, Dana (Lorraine Toussaint), and dad, Stewart (Bruce Davison). “He has a secret, which is going to come out at the party,” Johnson says. Drama aside, the Fosters will definitely get funky. “At the…