function mytheme_preprocess_page(&$vars){

 $css = $vars['css'];

//core modules

 unset($css['all']['module']['modules/node/node.css']);

//contrib

 unset($css['all']['module']['sites/all/modules/contrib/filefield/filefield.css']);

//custom

unset($css['all']['module']['sites/all/modules/custom/mymodule/mymodule.css']);

//theme css

unset($css['all']['theme']['sites/all/themes/mytheme/mytheme.css']);

 $vars['styles'] = drupal_get_css($css);

}

function mymodule_css_alter(&$css) {

  $exclude = array(

 //core

   'modules/system/maintenance.css' => FALSE,

//contrib

    'sites/all/modules/contrib/module/module.css' => FALSE,

//custom

    'sites/all/modules/custom/module/module.css' => FALSE,

//theme

    'sites/all/themes/mytheme/mytheme.css' => FALSE,

  );

  $css = array_diff_key($css, $exclude);

}