Themes make your game more beautiful. The more Levels you get, the more Themes you unlock. absolutely, you have to pay 500 Gold to purchase one of them, but, if your on the public testing server then you can get themes for free! Purchasing a theme is for setting it in your current game only. If you want to change of Themes, your current Theme will be removed and lost!
By purchasing a theme, all skyboxes are disabled and you're not able to place them anymore until the Theme will be removed.All themes are entirely customizable! You can change the sky color, the size of clouds, the size of the Moon, the size of the sun, the size of stars, their blinking speed and a lot more.