How to change GTK+ theme?

I am new in ubuntu, i have installed a large set of themes in ubuntu, i have also installed advanced-system-settings but when i want to change theme in advaced system settings , themes are not in drop down list?

You can use gnome-tweak-tool to change the GTK+ theme setting. To use gnome-tweak-tool, run the following command in a terminal:

sudo apt-get install gnome-tweak-tool

Note that installing gnome-tweak-tool also installs gnome-shell.

Search for gnome-tweak-tool or Advanced Settings (as it is named) in the Unity dash and open the tool.

Move to 'Theme' option on the left sidebar. You can now change the GTK+ theme by changing the options in the dropdown list. Check the image for clarification.