:Root, classes and their variables?

What change does solving this challenge do?

What is the difference between variables in :root comparing to in class?

In the link above, in every class there is “background: var(–penguin-belly, white);”
deleting/changing “white” to another color does nothing, what is the point of addressing the color?

Why is “–penguin-belly” being used in every class as background? is it because the other parts have the same color as the belly?