Kirki is a Customizer toolkit for WordPress. An open-source initiative by Themeum for WordPress theme developers.
Kirki allows you to build themes faster and more easily. With over 30 custom controls—ranging from simple sliders to advanced typography controls with Google Fonts integration—and features like automatic CSS generation and postMessage support, Kirki makes theme development a breeze.
You can find detailed documentation on how to use Kirki on https://www.themeum.com/kirki/