There isn't automated way for installation. In example, in KDE user can download and install desired theme in one click. In Unity, you must download desired theme manually, place it in right directory (manually again) and then you can set it as your theme. Well, let's take a tour...
Sites from which you can download themes are:
Choose desired theme(s), download it and extract it in the same directory. Now you have two options. You may set the theme only for one user (you) or for all users (system wide).
One userYou need to move extracted folder to ~/.themes folder. That folder is hidden and it located in your Home directory (~). To see hidden folders and files, press ctrl+H. Now, simply move or copy extracetd theme folder to the hidden one.
System wideIn this case, you'll need to move theme folder to /usr/share/themes. That folder is located, not in your Home, but in root directory (/). To do whatever in /, you must have administrative privileges, you must be root user. This can be done in two ways, GUI and CLI.
GUI way means that you need to open Nautilus as root and then move or copy theme folder to /usr/share/themes. Let's do this:
Press alt+F2 and type: gksu nautilus; press ENTER; enter your root password when prompted and simply move or copy theme folder to /usr/share/themes.
CLI way means that you'll need to do this from terminal. Open terminal and:
cd /location/of/downloaded/theme/folder sudo mv THEME_FOLDER /usr/share/themes/
Replace THEME_FOLDER with appropriate folder name.
Now you can use one of the tools we mentioned in article about Unity tweaking. Find themes section and installed theme(s) will be in a list of available themes. Just select it and that's all.