[Sagobal]

I'm Using 5.1.10 and it seems that there are Changes:
For full Icon Overlay I had to Use "display_contenttype_threadview_header" Template.
In this Template after

{vb:set postStatus, ''}

Insert

<vb:if condition="$user['lastvisit'] <= $conversation['created']"><div class="z-unread-icon"><img src="{vb:raw vboptions.bburl}/.././images/custom/unread-whatchamajigger4.png" alt="New since your last visit" title="New since your last visit" /></div></vb:if>

For Background/Spartan the Right Template should also be "display_contenttype_threadview_header"
I got it to work there, but NOT on the first Post of a thread. (Seens to be something with the Headline)
Find an Replace

<div class="b-post__body h-restore--on-preview h-padding-horiz-xxl h-padding-top-xl h-padding-bottom-m h-clearfix">

for Background with

<div class="b-post__body h-restore--on-preview h-padding-horiz-xxl h-padding-top-xl h-padding-bottom-m h-clearfix <vb:if condition="$user['lastvisit'] <= $conversation['created']">z-unread-bg-icon</vb:if>">

or for Spartan with

<div class="b-post__body h-restore--on-preview h-padding-horiz-xxl h-padding-top-xl h-padding-bottom-m h-clearfix <vb:if condition="$user['lastvisit'] <= $conversation['created']">-unread-bg-spartan</vb:if>">

Maybe this may help you all. And maybe there is someone who can figure out, how to use it on the first post.
Greetz