The Crate Attuner is a machine in the MCC Island hub, located in Splinters Workshop, where you can attune cosmetic crates to guarantee a cosmetic from a certain theme. The current available themes are Magical, Mechanical, Oceanic and Nature.
To attune a crate, open the menu and select a cosmetic pool and a crate to attune. If you have at least three Attunement Catalysts, the attunement will go through and the catalysts will be consumed.