facebook plugin box divider class border override

I am trying to get rid of the border-top on this widget and have tried to override it in custom.css but it will not override. Here is the css in like_box.php but I cannot find this file

.pluginBoxDivider {
		border-top: 1px solid #d8dfea;
}

So in custom.css I have tried this so far but it does not work

.pluginSkinDark .mhs.pluginBoxDivider {
		width:0 !important;
  		height: 0 !important;
  		border-top: 0;
  		border:none;
}

Any help would be appreciated, thanks!!

Here is the link to the site

https://wordpress.org/plugins/wordpress-facebook-like-plugin/