Edit index.main.php , line 25
If you have other files in your skin folder ending on .main.php edit them too.

This script adds target="_blank" to all [u]external [/u]links

skin_init( $disp );

require_js( '#jquery#' );

add_headline('<script type="text/javascript">
/* <![CDATA[ */
	
	jQuery(document).ready(function() {
		jQuery("a[href^=\'http:\']").not("[href*=\'" + window.location.host + "\']").attr(\'target\',\'_blank\');
	});
	
/* ]]> */
</script>');