It is worth noting the lack of darkness in the tower, at least at the ground floor. Radiant darkness behaves much like radiant light. While there was nothing out in the open to stop the darkness high atop Sunlit Tower from overcoming the courtyard Howard and Alex were just in, Sunlit Tower's ground floor has numerous floors of stone and wood in the way. Hence, until they get closer to the top (and somebody bothers to open the door to the rooftop auditorium where all this is happening), the ambient light in the tower is more than sufficient.

It is also worth noting that the journal Alex and Lucien found (which Marzos now has) also detailed attempts by the Founders to remove or nullify the poison from an ice rose. This is not what is causing the darkness on top of the tower.