Multisite: Duplicate Theme Customizer values across network

functions.php

<?php
/**
 * Duplicate specific Theme Customizer values across Multisite network
 *
 */

function my_theme_customize_multisite_duplicate() {
	
	if ( is_multisite() ) {
		// Get existing values from current blog as array
		$duplicated_values = array(
			'customizer_value_1',
			'customizer_value_2',
			'customizer_value_3',
		);

		foreach ( $duplicated_values as $value ) {
			$duplicates[] = get_theme_mod( $value ); // Get duplicated theme_mod variables as array
		}
		
		$sites = get_sites(); // https://codex.wordpress.org/Function_Reference/get_sites
		
		foreach ( $sites as $site ) {
			$current_site = $site->blog_id;
			
			// Exclude current blog
			if ( get_current_blog_id() !== $current_site ) :
				switch_to_blog( $current_site );
				
				// Duplicate previously defined values from other blog
				foreach ( $duplicated_values as $key => $value ) {
					set_theme_mod( $value, $duplicates[$key] ); // Set duplicated theme_mod variables from array
				}
				
				restore_current_blog();
			endif;
		}
	}
	
}
add_action( 'customize_save_after', 'my_theme_customize_multisite_duplicate', 99 );