I am pretty sure the only way to get the Tropical theme is to buy this package at 9.99 or the disk version. The theme is great and for you it would be worth the price. You could wait for a sale, but the theme is already my favorite theme. Season Mode and Tropical theme appears to be the only thing included in this DLC, the Steam store page says...
* Season Mode
* Tropical Theme
* Small optimizations
Buy it just for the Tropical Theme, I would recommend it to anyone who plays the game a lot.