What we are missing is built-in easy way to install themes in Ubuntu. Different with the old Ubuntu (versions before 11.04), now it is more tricky to tweak appearance of Ubuntu. But there is a convenient way starting at Ubuntu 14.04 Trusty Tahr named Unity Tweak Tool. With this tool, we can easily change our Ubuntu theme.
Install Unity Tweak Tool.See our Unity Tweak Tool article. The only reason we choose Unity Tweak Tools is because it is available for version Trusty until Wily in the official repository while tualatrix's Ubuntu Tweak is not.
Create .themes DirectoryCreate a ~/.themes folder. This is a hidden folder named .themes inside your $HOME folder. The most reliable way to do this is by executing command mkdir ~/.themes.
Copy Theme Folder into .themes FolderSuppose you have Mona 3.0 GTK theme from gnome-look.org. Mona is archived in .tar.gz format, so extract it in current folder so you can see a folder named 168447-Mona 3.0 GTK Theme.
Inside this folder, you will find two folders, Firefox Fix and Mona 3.0. You must see directory structure inside this folders, make sure you find a folder contains one or all of these important folders gtk-2.0, gtk-3.0, metacity-1, and xfwm4. They are Mona's theme folders. So, copy the parent of these folders (it is Mona 3.0) into ~/.themes folder you've created. So you will see the structure for Mona 3.0 GTK theme is ~/.themes/Mona 3.0/gtk-3.0/. This copying simply is the theme installation. This instruction apply into all other GTK themes.