No, they are not. This is the official law in Alabama.
"Anything that is not the factory lighting is illegal."
And since they don't come from the factory with under glow, it isn't legal. That is the official stance.
As long as you aren't displaying red or blue, the bulbs aren't visible, and it's never on while driving, they probably won't say much about it. White or amber on both sides near the running board area would be seen as courtesy lights probably.
Car shows, no problem, turn them on and show them off. Parking lots, maybe, except the officer on patrol might decide you are distracting other drivers, and parked on the road it almost certain he would decide that.
Frankly, I wouldn't spend the cash with all the negative possibilities you face.