Costume Sets are a type of bonus effect similar to Cookie Bonds that are activated by collecting certain Costumes. Alongside these sets are short stories explaining the context behind the Costumes, giving backstory to the different sets. The buffs provided through these Costume Sets automatically apply to all of a player's Cookies with more bonuses given for each Costume in a set that the player owns, and the individual bonuses from each level of a Set stack and add up.
Unlike Cookie Bonds, the bonuses provided do not depend on the Cookie's promotion level nor whether the player even owns the Cookie for which a Costume is made. Simply owning the appropriate Costume grants a passive bonus.
This buff-addition process repeats for each different Costume Set, and for each Set it occurs independently from other Bonds. All of these buffs stack as well.
List of Costume Sets