From what I've read careful programming. They also used images for buttons and other widgets. Pre-canned images look great but are also fixed in size, shape.

One of my goals with Merenda is to make a NextStep/Cocoa like GUI system but with a richer primitives - things like resizing corners dynamically with the shadows or adjusting gloss positions on skeuomorphic buttons.

Things web pages can do nowadays but in a NextStep like framework with rich MVC. Also with message passing but in Nim a more strongly typed language than ObjC.

An alternative evolution of Cocoa instead of SwiftUI.