Battery-powered lights and plug-in lights run from electrical outlets will work in a safe. Almost all gun safes have a small port in the lower back panel for running an electrical cord inside.
Incandescent or fluorescent lights work well in a safe but create safety hazards. Firearms are made partially of hard metal, and larger ones are often unwieldy, so a small tap with a metal barrel on a light bulb may shatter it. An LED light, however, uses much less electricity and is immune to breaking from a simple bump.
Lights work best when placed in the front of the safe, pointing toward the back. This keeps the glare out of your eyes when looking into the safe, and ensures that all the firearms are lighted.
Keeping a flashlight on a gun safe's shelf is always a good safety precaution. If the power is out throughout the home and you need to have access to the firearms, an independent light will allow you to still see the safe's interior.