I don't think you understand. Having a thread title appear in bold isn't something you choose. It is a way for the forum to notify you of threads you have viewed/read.
If you have written a thread/post then you must have read it, so it won't be in bold on your screen. However that thread title will show in bold to anyone that hasn't viewed/read it.
It will also appear in bold if there are replies/posts within the thread that you have not viewed/read.
Hope this helps x