From the packaging:

Now it's easy to customize Fluxx!

All you need is a Fluxx deck, a permanent marker, and a little bit of imagination, and you can add your own favorite concepts and wacky game ideas to the card game with ever changing rules!

[Bonus! Chrononauts expansion card inside!]

Note: The current edition of Fluxx Blanxx has one of each type of card (including the Creeper but not the Ungoal) and it does not have the bonus Chrononauts card.

Part of the Fluxx series.