It looks like Stern used a combination of lightboards (LEDs) and LED bulbs for the GI.
http://www.sternpinball.com/upload/games/game-of-thrones/pro/1888/GameOfThrones_Pro_web.pdf
Looks like bulbs in the backpanel (#88) and boards in the playfield GI. (#86,#87), so it doesn;t look like you can replace the playfield GI, just the backpanel GI with different bulbs.
Quoted from fosaisu:My only issue with the lighting is that they went with cool white instead of warm white for the GI (fantasy setting calls for warm). I also agree with the prior posters that the barage of flashes at the start of and during multiball is a bit much for player comfort, but I'd guess that'll be tweakable if it's not already. And the GI is swappable (right?)