wordpress plugin admin page style external file

wordpress-plugin-admin-styling.php

// In order to use an external css file in the plugin's admin panel: 
// 1. Copy following code to the main php file of your plugin (plugins/your_plugin/your_plugin_name.php)
// 2. Create admin.css (or adjust the name below)

function plugin_name_admin_css()
{
  $siteurl = get_option('siteurl');
  $url = $siteurl . '/wp-content/plugins/' . basename(dirname(__FILE__)) . '/admin.css';
  echo "<link rel='stylesheet' type='text/css' href='$url' />\n";
}

add_action('admin_head', 'plugin_name_admin_css');