found it myself, it's a quick and dirty patch:

in file "controllers\Index.php" in function "loadItems"

change

if($itemDao->hasMore())
$itemsHtml .= '<div class="stream-more"><span>'. \F3::get('lang_more').'</span></div>';

to

if($itemDao->hasMore()) {
$itemsHtml .= '<div class="stream-more"><span>'. \F3::get('lang_more').'</span></div>';
}
else {
$itemsHtml .= '<div id="nav-mark" onclick="$(\'#nav-mark\').click();">' . \F3::get('lang_markread') . '</div>';
}

This will add a "mark read" button at the end of each list of items.