I found that Peppermint 5 does not have screen savers - it uses a program called light-locker instead. But what if you prefer having a screen saver? Here is how to do it:
Click Menu >> Accessories >> Terminal
sudo apt purge light-locker light-locker-settings
type your sudo password and press <ENTER> again
then type in:
sudo apt install xscreensaver xscreensaver-gl xscreensaver-gl-extra xscreensaver-data-extra
and press <ENTER>It will install.
Now, let's make it autorun:
Open a Terminal, Menu >> Accessories >> Terminal
sudo gedit /etc/xdg/autostart/screensaver.desktop
press <ENTER>, type in your sudo password, press <ENTER>
and then place inside it the following information:
Save and close the file.
Choose a screensaver that you like:
To activate go to Menu >> System >> Screensaver
(to make things easier, you can copy and paste commands into the Terminal, after copying you can right click inside Terminal then left click on paste)