I want a pitch black cavern that can only be explored with a torch. How (specifically) do i turn cell lighting off?

right click on the cell and go to edit, then change the lighting colors to black.

Don't forget the directional lighting tab.

You could also try to create a new template. In Ck (Object Window) >World Data >Lighting Template >Right click on any in list and select new. Mess around with colours, fog, distances etc. Just give your template a new original name, this will help as if editing a pre-existing template your also editing other cells in the game, and thus may lead to conflicts with lighting overhaul mods.

A useful button in the editing window > Directional Ambient Values >[Set from Ambient] when ever you adjust ambient I tended to hit this button just set some easy base values. I have found that this needs to be done atleast once on initial experimenting as you'll find some weird results if you keep Direction Ambient ticked in cell edit>lighting tab.

Oh and right click in the Cell Pane>Cell name editing>edit..>Lighting Tab >Dropdown menu select your newly created template.

Have a look at some other templates while there for example distances, fog volume, etc, to add that little more atmosphere when you do have a torch lit. As a side note and something I'm currently doing for the mod I'm making, adding If your interested about having torch holders on some walls but with torches initially disabled, check out this unfinished article with CK wiki > http://www.creationkit.com/Additional_Prefabs

Happy modding