Art and Life
One of the major themes in the film is that of art and life and how they interact and coexist. Movie making was relatively new at the time, so very little detail was known by the public as to how films were made. Vertov shows us the inner working of creating work for the cinema and how real life is the subject for all art, even this new form coming on to the scene.
Process
Vertov creates a thematic of revealing the process of making the film during the picture. This theme plays out when we see the camera show the audience how focus works (with flowers going in and out of focus), as well as aperture and its function being related to shutters. And he reveals how the lens of the camera is in fact like the human eye capturing life.
Life
A major theme of this film is simply life. Capturing the day in the life of a city and the people who inhabit it. From them sleeping to their progress into their jobs, how they get around the city, how they enjoy themselves and escape. All of this in a period of a day captured by the cameraman.