Look: Penguins, Bruins unveil 2023 Winter Classic uniforms

That look is a reference to the old Pittsburgh Pirates, the cities original NHL team in the 1920s. Given that the Pirates are also the name of Pittsburgh's baseball team, it is a fitting look for the game being played in a baseball stadium. 

As for the host Bruins...

The Bruins' uniform borrows from a bunch of different eras of Boston hockey.

From the Bruins' official announcement: 

The big bold gold stripes on a black base make this sweater unmistakably Boston. There are touches of Vintage White, a light beige color meant to evoke aged textile as if this garment was found in a trunk deep in the back of an old skate shop, in the embellishments.
 The BOSTON wordmark is a custom typeface created by adidas and inspired by the original spoked-B worn in 1948. The arched form of the wordmark is a subtle classic varsity reference and nod to New England amateur hockey. The Boston bear logo is best known as the Bruins' secondary logo and uniform shoulder patch beginning in 1976 and worn through the early 1990s. The bear is recolored and paired with the BOSTON arching wordmark to create a complete city name and mascot creative lock-up.

The Penguins and Bruins have been regulars in the league's outdoor games. This is the second time Fenway Park has hosted the annual January game.