If you uninstall the overlay scrollbars as I recommended in an earlier post, you will not be able to start-up the Unity Tweak Tool. It will tell you that a schema is missing, saying that you need com.canonical.desktop.interface. You will wonder what the heck that is. Well it is part of the overlay scrollbars. You have to reinstall them.
Open a Terminal and copy and paste the following, then press enter:
sudo apt-get install overlay-scrollbar
Now the Unity Tweak Tool will launch. But now you’re back to those horrible scrollbars. You can actually fix this in the Unity Tweak Tool itself. On the very bottom of the page you will see Scrolling, click into that. Then just click Legacy. That’s all there is to it.