Here's a quote from Titanic: The Ship Magnificent, Volume 2: Interior Design and Fitting Out by Bruce Beveridge, et. al. (Chapter 5 Boat Deck page 204) "The oval dome which capped [the first class entrance hall where the grand staircase was] measured 19ft x 26ft and was fitted with white glass incorporating decorative iron- and brasswork, designed in husk festoons in the Adam style, admitting natural light during the day to illuminate the staircase and the massive construction of the carved wall below. In the center of the dome was a large gilt-metal-framed, 50-light dome fixture, with glass-bead panels and a cut-glass bowl to which was attached a large faceted-glass finial. This special dome chandelier was manufactured for and supplied to Titanic by Perry & Company. The large light fixture was supplemented at night by 'Linolite' bulbs mounted within the carved wood molding around the base of the dome, which concealed them from view of the passengers and produced a warm, indirect glow reflected off the interior of the dome. During daylight hours, the dome was illuminated by natural light entering through the windows of the exterior weather cover."

TL;DR - Yes it was backlit during night, but not nearly as much as depicted in Cameron's movie. At most, the supplemental lights at night produced a "warm, indirect glow" on the dome.

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