Theme decks are basically preconstructed decks to play the pokemon TCG with. They are intended to be used to learn to play the game, but they can just be fun to mess around with at times, doing theme deck only matches. They also come with everything you need to get started and some unique cracked ices variants of Mewtwo and Zeraora. Theme decks aren't really viable in a competitive setting (Unless it's restricted to theme decks), but they can still be worth checking out, especially if you want to get into the TCG.