I downloaded and installed the Adwaita Dark theme. It worked, but I had an issue. The title bar was supposed to turn black too, but it doesn't. I followed the steps in this OMG! Ubuntu article. There is a screenshot in the site with the title bars black. And here is a screenshot of my Ubuntu.
As you can see, I applied the Adwaita theme in "Window theme" too but it stays just like the default Adwaita.
Why does that happen and what can I do to fix it?