To create a CSS variable, you just need to give it a name with two hyphens in front of it and assign it a value like this:
--penguin-skin: gray;
This will create a variable named--penguin-skinand assign it the value ofgray. Now you can use that variable elsewhere in your CSS to change the value of other elements to gray.
TEST
In thepenguinclass, create a variable name--penguin-skinand give it a value ofgray.