I can read a dark-on-light text at normal size (on a retina display) for an hour without feeling my eyes are hurting, but I literally cannot read light-on-dark

I can read a dark-on-light text at normal size (on a retina display) for an hour without feeling my eyes are hurting, but I literally cannot read light-on-dark text more than 10 minutes and have to bump the font size one or two points...