I go into settings -> Power Management, trying to change the "Put display to sleep when inactive for:" and "Dim display when idle" settings. The changed settings are saved -- if I come back to the same menu, they appear as I left them.
But when I watch a full screen video, the display does get dim every once in a while. And if I leave my laptop alone for 15 minutes, the screen does go black (and lock itself).
How can I actually change these settings? I would also want to stop it from locking itself every time it went inactive -- just blank the screen.