You should almost be able to steer with just the pressure from your palms. First example that came to me of a light grip on the wheel was Louie Pagano. I’ve noticed he has a particular grasp on the wheel that indicates to me that he probably has very relaxed hands on the wheel. Check out this picture:
He holds the wheel low and not even all his fingers are wrapped around the wheel. His hands look really relaxed. You can see his thumb and fingers don’t even touch or wrap all the way around the wheel; he relies on his palm between his thumb and forefinger to press into the wheel. Or at least that how it appears to me. And obviously he’s damn quick being one of the top ranked TaG drivers in the country.
Overall, the more relaxed and comfortable you are in the kart, the better you’ll be as a driver. You can focus on racing rather than your grip on the wheel or your body flopping in the seat and you’ll be using your energy for the actual driving and mental focusing rather than keeping your body in place. The best posture in a kart is relaxed but stable.