Iterate through all active (base) themes in _bootstrap_glyphicons_supported()

My setup:
sites/all/themes/bootstrap/ - original bootstrap theme.
sites/all/themes/mybasetheme/ - intermediate base theme, based on bootstrap
sites/all/themes/mybasetheme/bootstrap/ - bootstrap css and assets
sites/all/themes/mytheme/ - site-specific theme, based on mybasetheme.

Problem: Bootstrap icons do not work.

Why?

function bootstrap_icon_bundles() {
  $theme = variable_get('theme_default', $GLOBALS['theme']);
  $cdn = theme_get_setting('bootstrap_cdn', $theme);
  $bundles = array();
  if ($cdn || (!$cdn && file_exists(drupal_get_path('theme', $theme) . '/bootstrap/fonts/glyphicons-halflings-regular.ttf'))) {
    ...

So.. it only works if the /bootstrap/ is in the site's default theme, but not if it is in any base theme of the default theme.