This season, the little black dress is de rigueur.
According to strictness (of etiquette, rule, or the like); obligatory; strictly required
If you say that a possession or habit is de rigueur, you mean that it is fashionable and therefore necessary for anyone who wants to avoid being considered unfashionable. T-shirts now seem almost de rigueur in the West End. considered to be necessary if you want to be accepted, fashionable etc - used humorously
required by etiquette or usage or fashion; "instruction as to when and where a silk hat is de rigueur