In tweak tool, go to Shell extensions and switch on "User themes"
If user themes is greyed out then follow instructions below which I found somewhere on web a while ago:
If you look in /usr/share/gnome-shell/extensions There are directories for your extensions. In each of the folders under the extensions folder, there will be a metadata.json file
There will be a line in that file that has "shell-version": [ "3.1" ],
Just change the version to 3.2 (or whatever version of gnome you are on) and then restart gnome-shell and see if your extension works.
Hope that works for you.
Normally you extract Gtk themes into .themes in your home directory