Group sub sub theme CSS separately I have a base, sub theme, and sub sub theme. I don't want the sub sub themes CSS to be grouped to CSS_THEME due to support for legacy IE browsers and the issues associated with large CSS files.

$value['group'] = 100 = CSS_THEME = (Theme-Layer CSS)
$value['group'] = 0 = CSS_DEFAULT = (Module-Layer CSS)
$value['group'] = -100 = CSS_SYSTEM = (System-Layer CSS)


Possible to add to this with a different weighting?

$value['group'] = 200 = CSS_SUB_THEME = (Sub Theme-Layer CSS)


Adding CSS to separate group.

  // Add sub sub theme css to separate group.
  drupal_add_css(PATH_TO_THEME/sub_sub_theme.css',
    array(
      'group' =&gt; CSS_SUB_THEME,
    )
  ); 


NOTE: not looking for related answers to legacy IE browser limitations as solutions such as css_splitter are already in place, simply can the above be done or is there a good reason you can't.
May not be the answer you're looking for but have you considered Advanced CSS/JS Aggregation as one of the features is "Prevent more than 4095 CSS selectors in an aggregated CSS file (IE 6-9 limitation)"