thank you for your baby.
Medical science has yet to isolate genes for fairness.
There is absolutely no medicine / drink / therapy / recipe for having a fair or a dark complexioned child.
You obviously know that it is dependent mainly on parental complexion.
Drinking coffee in moderation is fine in pregnancy
, and will affect the skin colour of the baby in no way.
Enjoy your pregnancy, and look forwards to a healthy and happy child, rather than a fair or dark one.