I have freeBSD 11.1 (i386). Tell me what program the keeper of the screen put?
I think it uses xscreensaver by default. There should be a way to set it up off of a Settings or Preferences (I’m not running Lumina so am going by memory).
The Lumina package should already install/run xscreensaver for you automatically (one of the core dependencies - kind of like Fluxbox). If you go into the applications menu you should see a “Screensaver” entry under the “Settings” category - that is the utility for configuring xscreensaver.