Why is Shell theme disabled in Gnome Tweak Tool?

I solved this problem myself, and it was an extremely trivial solution:

1. Open Gnome Tweak Tool.
2. Click on the Extensions menu item, and move the User themes slider to On.
3. Close Gnome Tweak Tool and open it again.
4. You should now be able to choose a Shell theme in the Appearance menu.

In addition to @miceagol's answer: If you do not see User Themes in the Extensions panel of GNOME Tweak Tool, you may need to install it from extensions.gnome.org. Restart the tweak tool after installing.

You can set a theme using Terminal :

 sudo chmod -R 755 /usr/share/themes/Zukitwo
gsettings set org.gnome.desktop.interface gtk-theme "Zukitwo"
gsettings set org.gnome.desktop.wm.preferences theme "Zukitwo"
gsettings set org.gnome.shell.extensions.user-theme name "Zukitwo"

use a known working default theme like above and it should get you back but this below may be your real nightmare are thems that won't work.Installing themes that don't work LEAVE behind entries and if you click on a failed entry you will break the tweak tool.

Uninstalling a themes not that easy though..No idea how to remove a bad theme from the list in tweak tool.