Try to add css into widget
plugins/nb40recenttopicsposts/widgets/nbRecentTopicsPosts.php
/* Return */
return $this->output($topics, $posts, $selectedTab, $tabs, $firstTab, $this->configuration, $this->uniqueKey, $widgetTitle);
change to
$css .= <<<CSS
<style>
.ipsWidget[data-blockid*="_nbRecentTopicsPosts_"] li.ipsDataItem.ipsDataItem_unread .ipsDataItem_title:before {
content: '\\f111';
font-family: 'FontAwesome';
font-size: 9px;
margin-right: 3px;
}
</style>
CSS;
/* Return */
return $this->output($topics, $posts, $selectedTab, $tabs, $firstTab, $this->configuration, $this->uniqueKey, $widgetTitle) . $css;