Answer code is not matched (typo) with instruction on _Basic CSS: Attach a Fallback value to a CSS Variable_
Try to answer with background: var(--penguin-skin, black); on both penguin-top and penguin-bottom. It will tell you that the answers are wrong. But, when you change the answer line to background: var(--pengiun-skin, black); The answers would be right.
Accepted answer is --pengiun-skin.
It should be --penguin-skin.
It's pengUIn, not pengIUn
It refers to variable --penguin-skin on class penguin.

I helped create these challenges - this was a deliberate typo to show how the fallback properties work
Error in current description:

Must be:
penguin class should declar the --penguin-skin variable and assign it to gray.
penguin class should declar the --penguin-belly variable and assign it to white.
penguin class should declar the --penguin-beak variable and assign it to orange.
^^ Yes there is a typo in description.
As of July 6, 2018 @ 10:30pm CST there is no longer a typo in either .penguin-top or .penguin-bottom class. As a result, the lesson does not display correctly, but the tests still pass.

Most helpful comment
https://learn.freecodecamp.org/responsive-web-design/basic-css/use-css-variables-to-change-several-elements-at-once/
Error in current description:
Must be:
penguin class should declar the
--penguin-skinvariable and assign it togray.penguin class should declar the
--penguin-bellyvariable and assign it towhite.penguin class should declar the
--penguin-beakvariable and assign it toorange.