I was under the impression it already did that automatically.
If it doesn't work for you, try using the forum navigational links rather than your browser's goback button. If you do use your browsers goback button, click refresh and it should tell you which has been read and which hasn't.
I know, and that's annoying, but if I don't want to read a topic (or accidentally press back on the browser), I don't want it to stay highlighted for a too long time.