I think the subject line only turns light blue if you click on the title of the thread.
If the thread has multiple pages, and you click on the latest page number, the thread title will not turn light blue.
Absolutely correct. It is a standard HTML/CSS feature - visited pages can have different colour. Thread links point to page one of the thread so if you've visited page one, it will turn light blue.
Clear your browser history if it's bothering you.