Step 1: On the home screen of Roundcube Webmail, the following image shows that the messages are marked as ‘read’ as the text is regular istead of bold.
Step 2: Highlight the message which you want to mark as ‘unread’
Step 3: Click on the following icon and them click on ‘unread’
Now, the selected message will be in bold which means it will show as ‘unread’