I don't think you can (unless you've seen anyone else doing it).
LOL ..
Could just be my computer ..
But nearly all the listed "topics" on any board appear in bold blue ..
Except for topics that I start and a few others ..
So, I just assumed that there was a way when creating a new topic to "bold" the "Subject" line ..
Triff ..
Of the "few others" that are not bolded, are these threads you have clicked on?
Click on a topic that is bolded and look at the thread, then return to the list of topics in that subforum and "refresh" your browser, it should not be bolded anymore. Bolded topics are topics you have not clicked on. I did say this in my prior post.