I think I know why now, I wasn't reading what you had wrote properly last night, (tiredness)
When you 'compile' a thread, it appears in bold, then when you post-other members see it in not bold 'per se' but dark, when they have entered that thread, like Jacky says it becomes 'lighter' in colour, this then shows you that you have 'read' it. If someone posts on it, then it goes back to being darker (for you to see that someone has left a reply).. then, once you have read it, then it goes lighter again and the cycle beings all over.
if you want something in particular to stand out in a thread title, then you can capitalise it- but not all of the thread title as then it will revert back to lower case. so a mixture is best.
should you wish to subscribe to any thread, then you need to goto 'thread tools' which is to the upper right at the top of every page, click on that and there is the option to 'subscribe to this thread', this can be a good way of 'remembering' which threads you have posted on and also help you in getting used to the forum. once you have subscribed to a particular thread, to access this, say if you have gone to work and need to to and remember where you posted, goto top right and click on 'settings' this then, takes you into your subscribed threads. (and this should also show you any new posts within those threads.)