To do this there are various applications like compiz, gnome tweak tool, my unity, ubuntu tweak ... try installing Ubuntu Tweak. For Ubuntu 12.04/11.10/11.04 just copy the following ppa and paste in the terminal (Ctrl+Alt+T) and run the commands one by one:

sudo add-apt-repository ppa:tualatrix/ppa 

sudo apt-get update

sudo apt-get install ubuntu-tweak

That installs the tualatrix PPA. Before installing this or other PPA's, it's advisable to read this so you understand the possible risks. (However, the tualatrix PPA is quite popular and its author is well-trusted in the community.)

• Are PPA's safe to add to my system and what are some "red flags" to watch out for?

After installation download themes from http://gnome-look.org/ and install...