I found a way to insert a custom style in my template, according to this page:
http://www.addthis.com/blog/2011/03/03/addthis-for-wordpress-plugin-version-2-0-2/#.T6Op0Os5Ld4
I added my style as an array in the plugin file (addthis_social_widget.php)
$addthis_new_styles = array(
'my_style' => array( 'src' => '<div class="addthis_toolbox addthis_default_style addthis_" %s ><a class="addthis_button_facebook"></a><a class="addthis_button_twitter"></a><a class="addthis_button_google_plusone_share"></a><a href="http://www.addthis.com/bookmark.php?v=250&pubid=ra-4fa382503d81907c" class="addthis_button_compact">
<a class="addthis_button_email"></a><a class="addthis_button_print"></a><a class="addthis_button_preferred_1"></a><a class="addthis_button_preferred_2"></a></div>', 'img' => 'toolbox-small.png', 'name' => 'My Style', 'above' => 'hidden ', 'below' => ''
), // My Style
And then added the code manually in the template with my style
<?php do_action('addthis_widget',get_permalink($post->ID), get_the_title($post->ID), 'my_style'); ?>