This cannot be changed. I suspect that this is a side-effect of the Office theme that you have chosen. Try the colorful theme.,

I've checked that. I tried all kinds of word themes. the issue still persist. I even changed the windows theme, It didn't work either. I was accustomed to seeing the color gray, but suddenly, it turned to black. The closest thing to this issue that I can recall, which may have caused this problem, is when I tinkered with the "colors" in the "Design" tab. I'm not entirely certain if it's due to that, but now, changing those colors doesn't seem to make any difference.