Costumes are cosmetic skins that are class specific, altering appearance only. Some are unlocked as you level the game, others are found in boxes or in the store.
How do you view available costumes and select them?
You can press C to open the character sheet, and locate the Costume icon, left clicking it will open the Collections tab for the specific classes costume.
Costumes that are greyed out have not been unlocked yet, left click on a costume with colour to select and apply it.
How do you obtain costumes?
Each class has 3 basic costumes that is available to everyone based on level, and several more purchasable in the Trove Store.
Starter Costume (Level 1)
Level 10 Costume (Level 10)
Max Level Costume (Level 25)
You can open the Trove store by:
Pressing F1 and selecting Store
Pressing Escape and selecting Store
You will have to add Credits to your account in order to buy a costume or box.
Mystery Boxes are cheaper but they unlock a random skin.