Responsive Web Design: Added hint to Create a Custom CSS Variable (#19605)

Added hint to Create a Custom CSS Variable (https://guide.freecodecamp.org/certifications/responsive-web-design/basic-css/create-a-custom-css-variable)

guide/english/certifications/responsive-web-design/basic-css/create-a-custom-css-variable/index.md

@@ -3,8 +3,19 @@ title: Create a custom CSS Variable
 ---
 ## Create a custom CSS Variable
 
-This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/certifications/responsive-web-design/basic-css/create-a-custom-css-variable/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>.
-
-<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
-
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+We need to create a variable name ```--penguin-skin``` and give it a value of ```gray``` in the ```penguin``` class.
+
+### Example
+
+```css
+--variable-name: value;
+```
+
+### Solution
+
+In the ```penguin``` class we create a variable name ```--penguin-skin``` and give it a value of ```gray```:
+
+```css
+--penguin-skin: gray;
+```