minecraft-assets/assets/minecraft/models/block/dark_prismarine_stairs_outer.json

{
  "parent": "minecraft:block/outer_stairs",
  "textures": {
    "bottom": "minecraft:block/dark_prismarine",
    "side": "minecraft:block/dark_prismarine",
    "top": "minecraft:block/dark_prismarine"
  }
}