How do I conditionally change the print.css stylesheet to use media = all I added this to my template.php, but it doesn't appear to be working.  I've also placed this code in theme_preprocess_html(), 

&lt;?php
  function theme_process_html(&amp;$variables) {
    if ($_GET['print'] == 'true'){
      $variables['css']['sites/all/themes/mytheme/css/print.css']['media'] = 'all';
    }
  }
?&gt;
I ended up using hook_css_alter(&amp;$css)

&lt;?php
  function MYMODULE_css_alter(&amp;$css) {
    if (isset($_GET['print']) &amp;&amp; $_GET['print'] == 'true') {
      $css[path_to_theme().'/css/print.css']['media'] = 'all';
    }
  }
?&gt;