how to add custom css as default for the whole site
If you want it on every theme, then you'll want it in a module.

Using this method you won't have to put the CSS in with every theme - just add and enable the module.

Just create a module that uses the following code:

function MYMODULE_preprocess_page($vars) {
  $options = array(
    'group' =&gt; CSS_THEME,
    'every_page' =&gt; TRUE
  );
  drupal_add_css(drupal_get_path('module', 'MYMODULE') . '/MYGLOBALCSS.css', $options);
  $vars['styles'] = drupal_get_css();
}


drupal_add_css()

drupal_get_css()

You'll need to clear your theme registry to have the changes picked up.