To create a CSS variable, a name must be given after two hyphens. The value then can be assigned.
--penguin-skin: gray;
Final Code
<style>.penguin{/* Only change code below this line */--penguin-skin:gray;/* Only change code above this line */position:relative;margin:auto;display:block;margin-top:5%;width:300px;height:300px;}.penguin-top{top:10%;left:25%;background:black;width:50%;height:45%;border-radius:70%70%60%60%;}.penguin-bottom{top:40%;left:23.5%;background:black;width:53%;height:45%;border-radius:70%70%100%100%;}.right-hand{top:0%;left:-5%;background:black;width:30%;height:60%;border-radius:30%30%120%30%;transform:rotate(45deg);z-index:-1;}.left-hand{top:0%;left:75%;background:black;width:30%;height:60%;border-radius:30%30%30%120%;transform:rotate(-45deg);z-index:-1;}.right-cheek{top:15%;left:35%;background:white;width:60%;height:70%;border-radius:70%70%60%60%;}.left-cheek{top:15%;left:5%;background:white;width:60%;height:70%;border-radius:70%70%60%60%;}.belly{top:60%;left:2.5%;background:white;width:95%;height:100%;border-radius:120%120%100%100%;}.right-feet{top:85%;left:60%;background:orange;width:15%;height:30%;border-radius:50%50%50%50%;transform:rotate(-80deg);z-index:-2222;}.left-feet{top:85%;left:25%;background:orange;width:15%;height:30%;border-radius:50%50%50%50%;transform:rotate(80deg);z-index:-2222;}.right-eye{top:45%;left:60%;background:black;width:15%;height:17%;border-radius:50%;}.left-eye{top:45%;left:25%;background:black;width:15%;height:17%;border-radius:50%;}.sparkle{top:25%;left:15%;background:white;width:35%;height:35%;border-radius:50%;}.blush-right{top:65%;left:15%;background:pink;width:15%;height:10%;border-radius:50%;}.blush-left{top:65%;left:70%;background:pink;width:15%;height:10%;border-radius:50%;}.beak-top{top:60%;left:40%;background:orange;width:20%;height:10%;border-radius:50%;}.beak-bottom{top:65%;left:42%;background:orange;width:16%;height:10%;border-radius:50%;}body{background:#c6faf1;}.penguin*{position:absolute;}</style><divclass="penguin"><divclass="penguin-bottom"><divclass="right-hand"></div><divclass="left-hand"></div><divclass="right-feet"></div><divclass="left-feet"></div></div><divclass="penguin-top"><divclass="right-cheek"></div><divclass="left-cheek"></div><divclass="belly"></div><divclass="right-eye"><divclass="sparkle"></div></div><divclass="left-eye"><divclass="sparkle"></div></div><divclass="blush-right"></div><divclass="blush-left"></div><divclass="beak-top"></div><divclass="beak-bottom"></div></div></div>