WordPress 3.4 introducted the Theme Customizer feature which allows site admins to tweak a theme’s settings and live preview those changes in real time.
In this post, I’ll give you a custom control class makes it possible to add a multiple select list to your theme customizer settings.
How to add the settings is not the focus of this post so you should check out posts like “How to leverage the Theme Customizer in your own themes” instead for great explaining on how to use the feature.
By default WordPress have controls for stuff like text fields, check boxes, color wheels, select lists, and image handling. But there’s no way to use a select list with the ‘multiple’ attribute unless you extend the WP_Customize_Control class.