replace css on certain pages by another css
You would need to use your theme to watch for it. You'll need to hook into preprocess_page and look for the global.css file in the css array and unset it. Below is an example of how to remove the modules/system/system.css you'll need to see where in the array global.css is and do the same thing. You'll want to wrap that inside of an if for your paths. In this example I use arg() to get the first path value (ie cart) and turn it on for that case. You can do what ever you want.

function phptemplate_preprocess_page(&amp;$vars)
{
    if(arg(0) == 'cart') {
        $css = $vars['css'];
        unset($css['all']['module']['modules/system/system.css']);
        $vars['styles'] = drupal_get_css($css);
    }
}