Since writing this documentation focus has been directed towards re-writing the Qt code leveraging the Qt QML framework. This will allow developers to create visually-superior, and easier to write and reason-about GUI code, whilst also lowering the barriers to entry for potential new developers who want to be able to focus on GUI code.

The recommendation therefore is to familiarise yourself with Qt QML and review the current codebase for the latest developments. You can follow along with the latest QML work in the specific bitcoin-core/qml-gui repo.