Adding external CSS files to your site head using 'drupal_add_css'

If you need to add an external CSS file to your D7 site, you could in theory just add it to the default html.tpl.php template. Sometimes this isn't ideal - like when using sub-themes - in this instance you can add your CSS via the template.php file. 
One particular scenario where this was useful was on a multisite setup, where I wanted to one generic css file included on all sites even though they each had their own Omega sub-theme. 

To inlcude external CSS in your template.php file use this:

function mytheme_preprocess_html(&$variables) {
  drupal_add_css('http://www.externalsite.com/styles.css', array('type' => 'external'));
}

BTW. If you want to add a new 'internal' CSS file to your site you should do it via your theme .info file - see this post on adding CSS the correct way. This will ensure that it's available for aggregation and optimisation with the site Performance settings.