Link : https://learn.freecodecamp.org/responsive-web-design/basic-css/use-a-custom-css-variable/
On left section, at the bottom :
Apply the --penguin-skin variable to the background property of the pengiun-top class.
Apply the --penguin-skin variable to the background property of the pengiun-bottom class.
On the code editor:
background: var(--**pengiun**-skin);
background: var(--**pengiun**-skin);