Recently I wanted to update a particular WordPress installation to use something other than the theme’s style.css file as it’s default CSS document.  Here is the code to enable that:

<?php
function modify_stylesheet_uri(){
$stylesheet_dir_uri = THEMEURI;
$stylesheet_uri = THEMEURI.'/css/styles-screen.css';
return $stylesheet_uri;
}
add_filter("stylesheet_uri", "modify_stylesheet_uri");
?>