Add custom endpoint to WP-Rest Api to retrive customizer values of active theme.

functions.php

<?php
add_action( 'rest_api_init', 'plugin_customizer_fields');

function plugin_customizer_fields() {
	register_rest_route( 'beyondthebell/v1', '/customizer-fields/', array(
		'methods' => 'GET',
		'callback' => 'get_active_theme_customizer_fields',
	) );
}

function get_active_theme_customizer_fields() {
	$current_active_theme = wp_get_theme();
	$current_active_theme_name = $current_active_theme->get('Name');
	$theme_mod = get_option( 'theme_mods_'.$current_active_theme_name );
	return $theme_mod;
}