Supporter costumes are a feature that players with the supporter status can use to alter their appearance and take form of many other beings in the game (e.g. skeleton) after purchasing the costume. These can be used with the costume command "/costume number" with number being any number between 0 to 148.
Due to the update from June 2018,  this article become outdated and needs revisions. To summarize for now, buying diamonds allows you to change your character's appearance but to use actual costumes you will need to purchase the costume with additional diamonds. After you purchase the costume for a separate amount of diamonds your account will use one diamond a day to maintain you wearing this costume.
For example, if you buy 30 diamonds and unlock a costume that costs 2 diamonds, you will own the costume and can wear it for another 28 days (30-2=28) or until you purchase more diamonds. You only have a purchase a costume once in order to have it unlocked and may use that costume whenever you have diamonds. The costume shop can be accessed via the appearance tab and will also show you what percentage of total costumes you have unlocked.
Become a Supporter and enjoy these Perks and more
List of supporter costumes (under construction, more added with time, feel free to contribute.)