Ok but now the icons background doesn't really match the windowbg background. If that bothers you, you could fix that by making a minor edit in both BoardIndex.template and MessageIndex.template files. Find the below in both files and replace .gif with .png:
if(!empty($modSettings['tidy_child_boards_icon']))
echo '<img src="', $settings['images_url'], '/', ($child['new'] ? 'on' : 'off'), '.gif" width="12" height="12" alt=""> ';