**Course Title:** Modern CSS: Responsive Design and Advanced Techniques **Section Title:** Custom Properties (CSS Variables) and Calc() **Topic:** Defining and using custom properties with the `--variable-name` syntax **Introduction** Custom properties, also known as CSS variables, are a game-changer in modern CSS development. They allow you to define reusable values that can be used throughout your stylesheet, making it easier to manage and maintain your code. In this topic, we'll explore how to define and use custom properties with the `--variable-name` syntax. **Why Use Custom Properties?** Custom properties offer several benefits, including: * **Consistency**: By defining a value in one place, you can ensure consistency throughout your stylesheet. * **Reusability**: Custom properties can be used multiple times, reducing code duplication and making maintenance easier. * **Easier updates**: If you need to update a value, you can do so in one place, without having to search and replace throughout your code. **Defining Custom Properties** Custom properties are defined using the `--variable-name` syntax, followed by a value. For example: ```css :root { --primary-color: #3498db; --font-size: 18px; } ``` In this example, we define two custom properties: `--primary-color` and `--font-size`. The `:root` pseudo-class is used to define global custom properties that can be accessed from any element. **Using Custom Properties** To use a custom property, you can reference it using the `var()` function, like this: ```css .button { background-color: var(--primary-color); font-size: var(--font-size); } ``` In this example, we use the `var()` function to reference the `--primary-color` and `--font-size` custom properties. The `var()` function takes the custom property name as an argument, and returns its value. **Example Use Cases** Custom properties are useful in a variety of scenarios, such as: * **Theming**: Define a set of colors and typography that can be used throughout your application. * **Spacing**: Define a set of spacing values that can be used for margins, padding, and borders. * **Responsive design**: Use custom properties to define breakpoints that can be used to conditionally apply styles. **Best Practices** Here are some best practices to keep in mind when working with custom properties: * **Use meaningful names**: Choose custom property names that are descriptive and easy to understand. * **Use the `:root` pseudo-class**: Define global custom properties using the `:root` pseudo-class. * **Use the `var()` function**: Use the `var()` function to reference custom properties. **Conclusion** Custom properties are a powerful tool in modern CSS development. By defining reusable values, you can make your code more consistent, reusable, and easier to maintain. In this topic, we've covered the basics of defining and using custom properties with the `--variable-name` syntax. For more information on custom properties, check out the [MDN Web Docs](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties). **What's Next?** In the next topic, we'll explore the `calc()` function and how it can be used for dynamic calculations. Stay tuned! **Leave a comment below if you have any questions or need further clarification on any of the concepts covered in this topic.**