Lost a Change of Color-Function, thus I often click and read it again, ....

Hi:
When I click one of posts/threads to read it, a color of that thread changes from 'blue' to 'grayish-blue.' For that change of color of the post/thread, I can easily remember that I finished to read it already and do not need it again.

However, since yesterday a color does not change from 'blue' to 'grayish-blue' on the post/thread I clicked and read it. Hence I often read the same post/thread again unless I can remember I read it already.

My question is:
How to restore 'change of color' after click and read one of posts, or several more posts?:confused:

Thanks for your help in advance.