I am using Ubuntu 15.10 desktop version. In system Appearance setting I can see three themes installed viz. Ambiance, Radiance, and High Contrast. While under /usr/share/themes directory, I can see many themes listed there viz. Adwaita, AgingGorilla, Ambiance, Atlanta, Bright, Crux, Default, Emacs, Esco, HighContrast, Metabox, Radiance, Raleigh, and Simple. So why all these themes are not listed under Appearance setting? Do I need to unlock the rest themes or am I mistaken somewhere? Thanks in advance!
A solution found in this thread. For those who're facing the same issue, you can install Unity Tweak Tool from Ubuntu Software Center. But unfortunately, it won't help much as it changes the icons only, not the overall appearance. :-(