[Remove core css] https://github.com/Krimson/wundertheme/blob/master/template.php #d7

template.php

/**
 * Implements hook_css_alter().
 */
function MY_THEME_css_alter(&$css) {

  /* Remove some default Drupal css */
  $exclude = array(
    'modules/aggregator/aggregator.css' => FALSE,
    'modules/block/block.css' => FALSE,
    'modules/book/book.css' => FALSE,
    'modules/comment/comment.css' => FALSE,
    'modules/dblog/dblog.css' => FALSE,
    'modules/field/theme/field.css' => FALSE,
    'modules/file/file.css' => FALSE,
    'modules/filter/filter.css' => FALSE,
    'modules/forum/forum.css' => FALSE,
    'modules/help/help.css' => FALSE,
    'modules/menu/menu.css' => FALSE,
    'modules/node/node.css' => FALSE,
    'modules/openid/openid.css' => FALSE,
    'modules/poll/poll.css' => FALSE,
    'modules/profile/profile.css' => FALSE,
    'modules/search/search.css' => FALSE,
    'modules/statistics/statistics.css' => FALSE,
    'modules/syslog/syslog.css' => FALSE,
    'modules/system/admin.css' => FALSE,
    'modules/system/maintenance.css' => FALSE,
    'modules/system/system.css' => FALSE,
    'modules/system/system.admin.css' => FALSE,
    'modules/system/system.maintenance.css' => FALSE,
    'modules/system/system.messages.css' => FALSE,
    'modules/system/system.theme.css' => FALSE,
    'modules/system/system.menus.css' => FALSE,
    'modules/taxonomy/taxonomy.css' => FALSE,
    'modules/tracker/tracker.css' => FALSE,
    'modules/update/update.css' => FALSE,
    'modules/user/user.css' => FALSE,
    // Remove contrib module CSS 
    drupal_get_path('module', 'views') . '/css/views.css' => FALSE,
  );

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

  /* Get rid of some default panel css */
  foreach ($css as $path => $meta) {
    if (strpos($path, 'threecol_33_34_33_stacked') !== FALSE || strpos($path, 'threecol_25_50_25_stacked') !== FALSE) {
      unset($css[$path]);
    }
  }
}