Decopunk is a recent subset of the punk sci-fi genres (specifically Dieselpunk), centered around the Art Deco and Streamline Moderne art styles that were popular during the Roaring 20s and stayed in fashion until sometime in the 1950s. It has a sleeker and shiny aesthetic compared to Dieselpunk, which has a tendency to be more gritty and dark aesthetic to it. Often times, Decopunk will use slightly more modern technology compared to the times it's supposed to evoke like VHS tapes. There are two schools of thought when it comes to Decopunk: Piecraftian and Ottensian
Piecraftian Decopunk tends to focus on the aesthetics popular during World War 1 and World War 2 and usually focuses on how human culture would evolve in the face of constant widespread warfare. This can cause Piecraftian Decopunk to bleed somewhat into Dieselpunk since the focus of Piecraftian Decopunk to be more on survival than aesthetic evolution. Examples of Piecraftian Decopunk could include movies like the original Mad Max, as well as George Orwell's 1984, Wolfenstein: New Order, and the PS3 exclusive Killzone.
Ottensian Decopunk chooses, instead, to focus on the aesthetics and decadence of the roaring twenties and seeing how it would evolve if it were completely uninterrupted by the 1929 Market Crash that led to the Great Depression or by either World War and generally has a more utopian vision of the future (sort of like how Solarpunk is in relation to Cyberpunk) and is more of a version of Retro-Futurism. Examples of fiction that work into the Ottensian Decopunk label include The Rocketeer, Sky Captain and the World of Tomorrow, and (retroactively), the works of Isaac Asimov.
Other works of fiction within the Decopunk scene include the works of H.P. Lovecraft and Noir fiction like Casablanca and LA Noire.
A lot of Decopunk fashoin does tend to draw upon Dieselpunk's similar influences, but there's definitely more of a focus on looking fashionable and classically presentable than there is on the grittiness one would find within the Dieselpunk genre, so definitely expect more zoot suits, dresses, and higher-end fashion in Decopunk circles than you would find within the Dieselpunk genre of aesthetics.