Nebula is a WordPress theme framework that focuses on enhancing development. The core features of Nebula make it a powerful tool for designing, developing, and analyzing WordPress websites consistently, yet its deliberately uncomplicated code syntax also serves as a learning resource for programmers themselves.
https://make.wordpress.org/core/2017/11/01/improvements-to-the-customize-js-api-in-4-9/
The Customizer PHP syntax for declaring controls has been simplified.