Each Planeswalker has access to theme decks. Theme decks are special pre-built decks that you build towards - you need all the cards from a theme deck to use it. Each theme deck is built to be usable, be it in early game, mid game or late game, depending on the deck. Themes decks are not purchasable outright - you only need to gather all the cards through card packs to complete them.
Initially, only the Kaladesh Planeswalkers have a theme deck for every stage of the game - older Planeswalkers currently only have a theme deck for the early game, featuring cards from their expansions. We will eventually fill out the decks for the older Planeswalkers, but we wanted to give all the attention to the current ones. As a little bonus, whenever you complete a theme deck, you’ll get a small reward, based on the deck tier. Late-game decks will grant Mana Jewels upon completion!