To select a theme, go to the Theme tab in Gnome tweak tool. Next to shell theme, you should see a button the says "None" with a folder icon. If you see a triangle with an exclamation point, it means that you have not installed the gnome-shell theme selector extension, which can be installed using the following commands in a terminal window:
sudo add-apt-repository ppa:ferramroberto/gnome3
sudo apt-get update
sudo apt-get install gnome-shell-extensions-common
sudo apt-get install gnome-shell-extensions-user-theme
Once the extension is installed, select the the .zip file for the theme that you want to install (Do NOT unpack it) by clicking on the "None" button in the Theme tab of gnome-tweak-tool. Then, select it from the list to the right of the "None" button.