Unread PM are highlighted by a specific icon. But it is not possible to build a style that presents the Unread PM with a different presentation other than this different icon. The reason is that no flag is available to the template to indicate that the PM is unread.
Note that this is not consistent with posts that have a flag S_UNREAD_POST set at line 1557 of viewtopic.php, or with topics that have a flag S_UNREAD_TOPIC set at line 662 of viewforum.php.
So I propose to add the following line:
'S_PM_UNREAD' => ($row['pm_unread']) ? true : false,
At line 172 of includes/ucp/ucp_pm_viewfolder.php
I know I should have provided a Unix GIT patch. But I use SVN under Windows. I did not feel it was worth to install a UNIX and a GIT client for a single line patch. In case it can be useful, attached is the SVN / Windows (with CRLF) patch file.