fix(guide): restructure curriculum guide articles (#36501)

* fix: restructure certifications guide articles
* fix: added 3 dashes line before prob expl
* fix: added 3 dashes line before hints
* fix: added 3 dashes line before solutions

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

@@ -1,21 +1,27 @@
 ---
 title: Create a custom CSS Variable
 ---
-## Create a custom CSS Variable
+# Create a custom CSS Variable
 
-<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+---
+## Problem Explanation
 We need to create a variable name ```--penguin-skin``` and give it a value of ```gray``` in the ```penguin``` class.
 
-### Example
+**Example:**
 
 ```css
 --variable-name: value;
 ```
 
-### Solution
+
+---
+## Solutions
+
+<details><summary>Solution 1 (Click to Show/Hide)</summary>
 
 In the ```penguin``` class we create a variable name ```--penguin-skin``` and give it a value of ```gray```:
 
 ```css
 --penguin-skin: gray;
 ```
+</details>