Drupal: Remove as much pre-generated core CSS as possible

preprocess.inc

<?php

/**
 * Implementation of hook_css_alter().
 */
function MYTHEME_css_alter(&$css) {

  // Remove core block stylesheet(s)
  unset($css[drupal_get_path('module', 'block') . '/block.css']);

  // Remove core date stylesheet(s)
  unset($css[drupal_get_path('module', 'date') . '/date_api/date.css']);

  // Remove core ctools stylesheet(s)
  unset($css[drupal_get_path('module', 'ctools') . '/css/ctools.css']);

  // Remove core fields stylesheet(s)
  unset($css[drupal_get_path('module', 'field') . '/theme/field.css']);

  // Remove core node stylesheet(s)
  unset($css[drupal_get_path('module', 'node') . '/node.css']);

  // Remove core system stylesheet(s)
  unset($css[drupal_get_path('module', 'system') . '/admin.css']);
  unset($css[drupal_get_path('module', 'system') . '/defaults.css']);
  unset($css[drupal_get_path('module', 'system') . '/system.css']);
  unset($css[drupal_get_path('module', 'system') . '/system.base.css']);
  unset($css[drupal_get_path('module', 'system') . '/system.menus.css']);
  unset($css[drupal_get_path('module', 'system') . '/system.messages.css']);
  unset($css[drupal_get_path('module', 'system') . '/system.theme.css']);

  // Remove core user stylesheet(s)
  unset($css[drupal_get_path('module', 'user') . '/user.css']);

  // Remove core views stylesheet(s)
  unset($css[drupal_get_path('module', 'views') . '/css/views.css']);
}