Caffeine content in coffee remains very stable during the roasting process, so caffeine content in dark and light roasts is essentially the same.The idea that light roasts have more caffeine is because, per bean, they’re denser, and so by volume, they do have more caffeine. By weight, they’re the same.
Coffee is ~98.5% water, and 1.5% coffee. Coffee is 1.2% caffeine, so in any given cup of coffee, caffeine is only 0.0168% of the beverage.
Caffeine content in coffee is much more dependent on the brew method than on roasting.