Gnome 2 had some wonderful tools that allowed you to change and customize the desktop themes the way you like. With the arrival of Gnome 3, among with other killer features, theme customizations was gone. If some user needed to customize his/her desktop theme, he would need to edit the configuration files which is out of capability of most users.
Thankfully, a new tool has been created by Satya164 from Deviantart, that will allow you to customize your GTK theme to some extent. This tool has limited functionality right now, like changing background color, change Xfce, Unity and Gnome panel background colors, and some menu and font colors, but hopefully, we can expect more functionality as the development proceeds.
You can install the tool by adding a PPA with these commands from your terminal:
sudo add-apt-repository ppa:shimmerproject/ppa
sudo apt-get update
sudo apt-get install gtk-theme-config