Die Standardfunktionen des WordPress Customizers:
https://maddisondesigns.com/2017/05/the-wordpress-customizer-a-developers-guide-part-1
Und einige sehr wertvolle Erweiterungen:
https://maddisondesigns.com/2017/05/the-wordpress-customizer-a-developers-guide-part-2/
Customize Image Gallery Control: Adds Customizer support for image gallery control type.
https://github.com/xwp/wp-customize-image-gallery-control