Play Roll The World between October 16th and November 2nd, and you will unlock two Halloween themed skins: The pumpkin skin and the skull skin. You will start off with the pumpkin skin, and the skull skin will be unlocked when you visit the paint shop in the abandoned lab. The main hub of the game has also been updated for a limited time during this event.
If you haven't played the game yet and are looking for a discount, the game will also go on sale during the Steam Halloween Sale between October 29th and November 2nd.