I presume by outside light you mean the default level ambient light?
If you wish that only actual placed lights illuminate your level - you can change the default ambient light settings in the settings.ini file for your game/level - changing the lighting setting to 0 (zero) or 1 (one) - that will start you with a level that is actually almost completely black so if you dont insert any light you wont be able to see anything much at all.
Actually - you may well have to edit the mygame settings.ini file and not your specific game.ini file if you have created one as there is a bug in FPSC that stops you changing setttings that will show up in editor test run if you dont edit the mygame.ini file itself.
While in editor test run mode in your level - hitting the < & > will change the ambient light in real time in your level test run so you can quickly test out what complete darkness looks like while you continue to add lights as you develop and see how it progresses if you want.
Although there is a bug I mentioned in regard to the editor -pereventing changing some settings in the build dialogue box that wont show up in test run - they will in game compile - thus the < & > are useful to use in test so you can see what your final build will look like if you dont want to edit the default mygame settings.ini file.
Your best bet is to make your level useing the <> keys to set ambient lighting in test run and then edit your game settings.ini manually following final game compile - changing build dialogue box settings in the .ini files can cause unstable levels or level corruption - another bug so watch that one.
You should always keep backups anyway so nothing can then go wrong - just put your copies back.
Hope that helps