Quick custom wp rest route

rest_route.php

/* Rest Routes */
function theme_data() {
    $obj = [];
    $obj["site_url"] = get_site_url();
    return $obj;
}

add_action( 'rest_api_init', function () {
    register_rest_route( 'theme', 'data', array(
      'methods' => 'GET',
      'callback' => 'theme_data',
    ) );
} );