From Halopedia, the Halo wiki
Steam rewards are attainable items awarded for players who play Halo games on Steam. They are commonly divided into Trading Cards, Emoticons and desktop wallpapers. Currently, the only Halo games to support Steam Rewards are Halo Wars: Definitive Edition, Halo: Spartan Assault and Halo: The Master Chief Collection.
The rewards for Halo: Spartan Assault can be found here.
The rewards for Halo Wars: Definitive Edition can be found here.
The rewards for Halo: The Master Chief Collection can be found here.