Well 71 kg at what height? And also it isn’t unhealthy to see the veins. I’m at a really healthy weight and my veins pop out basically all the time in my hands. And blood is blue so of course you’d see that.
When your body fat percentage goes under 10% that’s pretty low, but when it gets around 5-6% or lower then you certainly need to add weight.
Nothing abnormal about that. I have low body fat and I can see veins too. Not only on hands, but others parts of body also. Especially when I exercise or do yoga.