Currently there is no separate class for unread messages. I will try to add in next version. If you need urgently you can override
messagebox.php template to achieve this. Instruction to override template in https://www.shamimsplugins.com/docs/front-end-pm-pro/customization-front-end-pm-pro/change-templates/
Excerpt is intended to have same length ( 100 characters ). but some messages does not have that much characters. So it become uneven. You can use
fep_get_the_excerpt filter hook to return ever length as you want.