Put WordPress site into maintenance mode

functions.php

// Put site into maintenance mode
function eastsideCode_maintenance_mode(){
    if(!current_user_can('edit_themes') || !is_user_logged_in()){
        wp_die('<h1 style="color:purple">Website under Maintenance</h1><br />We are performing scheduled maintenance, and will be back online shortly. If you think you've received this notice in error, please contact your website administrator. Thanks!');
    }
}
add_action('get_header', 'eastsideCode_maintenance_mode');