Just be cute. Private area's like penthouse can be dressy for private guests and special occasions. Lower bar area is just very lively, upbeat and chilled, really relaxed chic. Jeans, to suits. Just do you!
Actually the Mastro's in Beverly Hills is pretty laid back specially on the floors with music. Haven't tried the restaurant downstairs though, not sure if people who eat there are more formal. I have doubts. Haven't tried the one in Malibu so, no idea there. In BH, you'll see people in sweaters, jeans, even sports clothes but I guess the McLarens, Ferraris and Rolls they get down from make up for it.
Dressy casual
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