I'd just like to confirm that, according to the Boeing Manuals, the 737NG does have the dome lights running off the Battery Bus (Battery Switch required). However, assuming we are talking about the two Dome lights at the back of the cockpit, the one to the left of the door has two bulbs. The larger of the two bulbs is powered by the Battery Bus under normal circumstances. The smaller bulb is powered by a rechargeable battery pack and is not normally illuminated. However, if the aircraft loses power and the emergency light switch is armed, this smaller bulb will be illuminated.
The battery pack, by the way, recharged by the Left Main DC Bus.
Hope this helps.
Rgds.
Q.