This tutorial will show you how add add/enable screensavers in Ubuntu 12.04.By default Ubuntu 12.04 only have gnome-screensaver installed,and your screen will appear black when system is idle,you can install xscreensaver to enable/add screensavers.
1.Open a terminal by pressing CTL+ALT+t
2.Uninstall gnome-screensaver using below command
sudo apt-get purge gnome-screensaver
3.Install xscreensaver using below command
sudo apt-get install xscreensaver xscreensaver-data-extra xscreensaver-gl-extra
4.Add xscreensaver to autostart file so screensaver daemon can run in background every time you log in.
sudo gedit /etc/xdg/autostart/screensaver.desktop
Then add below lines to this file
[Desktop Entry] Name=Screensaver Type=Applicaton Exec=xscreensaver -nosplash
5.Reboot then you can open Screensaver from Dash and customize it.