They basically work like a custom, you can f. e. buy a "Faerie Shoyru" custom and apply it to whichever of your Shoyrus you'd like, they don't have to be painted faerie. Only a select amount of colors are available right now (Faerie, Darigan, Grey are temporary, Usuki + MSPP are permanent). All UCs were automatically converted and received their custom automatically. If you have f. e. a badly named UC Grey Zafara you can now take that appearance off and give it to any other Zafara you might have. Effectively no UC exist anymore, it's now just converted pets with customs.
Customs are applied and removed in the styling chamber: https://www.neopets.com/stylingchamber/
Customs are bought in the NC Mall, they are currently discounted as the tweet says. 300 NC for a random custom or 700 NC to choose the color that you want.
But you can always choose for which species you want the custom, you have to actually choose which pet received the custom but it doesn't really matter.
