Plugin API/Action Reference/customize preview init
This action hook allows you to enqueue assets (such as javascript files) directly in the Theme Customizer only. To output saved settings onto your live site, you still need to output generated CSS using the wp_head hook.
Generally, this hook is used almost exclusively to enqueue a theme-customizer.js file for controlling live previews in WordPress’s Theme Customizer.
.. add_action( 'customize_preview_init', 'mytheme_customizer_live_preview' );