Supplying the building blocks is how you're able to compose them into something different. If all the UI libraries just gave you highly abstracted prebuilt com

Supplying the building blocks is how you're able to compose them into something different. If all the UI libraries just gave you highly abstracted prebuilt components with little to no customisability, then you wouldn't use them. Because as soon as you want to do something different, you'd have to write your own or find someone else's.