feat(curriculum): adding content to css variables review page (freeCodeCamp#57120)

Author: jdwilkin4

client/i18n/locales/english/intro.json

@@ -2251,7 +2251,8 @@
       "review-css-variables": {
         "title": "CSS Variables Review",
         "intro": [
-          "Review the CSS Variables concepts to prepare for the upcoming quiz."
+          "Before you are quizzed on the fundamentals of CSS variables, you first need to review.",
+          "Open up this page to review how to work with CSS custom properties(CSS variables) and the <code>@property</code> rule."
         ]
       },
       "quiz-css-variables": {

curriculum/challenges/english/25-front-end-development/review-css-variables/671a98fbaabfba994e3d9a7c.md

@@ -9,7 +9,75 @@ dashedName: review-css-variables
 
 Review the concepts below to prepare for the upcoming quiz.
 
+## CSS Custom Properties (CSS Variables)
 
+- **Definition**: CSS custom properties, also known as CSS variables, are entities defined by CSS authors that contain specific values to be reused throughout a document. They are a powerful feature that allows for more efficient, maintainable, and flexible stylesheets. Custom properties are particularly useful in creating themeable designs. You can define a set of properties for different themes:
+
+```css
+:root {
+  --bg-color: white;
+  --text-color: black;
+}
+
+.dark-theme {
+  --bg-color: #333;
+  --text-color: white;
+}
+
+body {
+  background-color: var(--bg-color);
+  color: var(--text-color);
+}
+```
+
+## The `@property` Rule
+
+- **Definition**: The `@property` rule is a powerful CSS feature that allows developers to define custom properties with greater control over their behavior, including how they animate and their initial values. 
+
+```css
+@property --property-name {
+  syntax: '<type>';
+  inherits: true | false;
+  initial-value: <value>;
+}
+```
+
+- **`--property-name`**: This is the name of the custom property you're defining. Like all custom properties, it must start with two dashes. `--property-name` can be things like `<color>`, `<length>`, `<number>`, `<percentage>`, or more complex types.
+- **`syntax`**: This defines the type of the property. 
+- **`inherits`**: This specifies whether the property should inherit its value from its parent element. 
+- **`initial-value`**: This sets the default value of the property.
+- **Gradient Example Using the `@property` Rule**: This example creates a gradient that smoothly animates when the element is hovered over.
+
+```html
+<div class="gradient-box"></div>
+```
+
+```css
+@property --gradient-angle {
+  syntax: "<angle>";
+  inherits: false;
+  initial-value: 0deg;
+}
+
+.gradient-box {
+  width: 100px;
+  height: 100px;
+  background: linear-gradient(var(--gradient-angle), red, blue);
+  transition: --gradient-angle 0.5s;
+}
+
+.gradient-box:hover {
+  --gradient-angle: 90deg;
+}
+```
+
+- **Fallbacks**: When using the custom property, you can provide a fallback value using the `var()` function, just as you would with standard custom properties:
+
+```css
+.button {
+  background-color: var(--main-color, #3498db);
+}
+```
 
 # --assignment--