CSS Declaring and Using CSS Custom Properties Step by step Implementation and Top 10 Questions and Answers
 Last Update:6/1/2025 12:00:00 AM     .NET School AI Teacher - SELECT ANY TEXT TO EXPLANATION.    15 mins read      Difficulty-Level: beginner

CSS Declaring and Using CSS Custom Properties

CSS Custom Properties, often referred to as CSS Variables, are a versatile and powerful feature introduced in CSS3 that allow developers to store values that can be reused throughout the stylesheet. This not only simplifies maintenance but also enhances the scalability and readability of the code.

Declaring CSS Custom Properties

CSS Custom Properties are declared within a selector, typically within the :root pseudo-class, which represents the highest-level element in the document. However, they can be declared within any selector, allowing for more granular control over which sections of the document can access and use them.

:root {
  --primary-color: #3498db;
  --secondary-color: #2ecc71;
  --font-size-base: 16px;
  --spacing-large: 20px;
}

In the above example, --primary-color, --secondary-color, --font-size-base, and --spacing-large are custom properties, indicated by the double hyphen (--) prefix. This prefix is essential for distinguishing custom properties from regular CSS properties.

Another common place to declare custom properties is within component-specific selectors:

.button {
  --button-bg-color: #e74c3c;
  --button-text-color: white;
}

This makes the properties scoped to only the elements matched by the selector.

Using CSS Custom Properties

Once declared, CSS Custom Properties can be used in place of static values by referencing them with the var() function. This function can take one or two arguments—the first being the name of the custom property, and the second being a fallback value that will be used if the variable is not defined.

body {
  background-color: var(--primary-color);
  font-size: var(--font-size-base);
  padding: var(--spacing-large);
}

In this snippet, the background-color will adopt the value stored in --primary-color, while font-size and padding will inherit their values from --font-size-base and --spacing-large, respectively.

You can also use custom properties within calc() functions and media queries, providing greater flexibility in styling.

.container {
  width: calc(100% - 2 * var(--spacing-large));
}

@media (min-width: 600px) {
  .container {
    width: calc(100% - 4 * var(--spacing-large));
  }
}

In media queries, custom properties can also be useful for maintaining consistency across stylesheets, as you can define common values once and use them throughout your styles.

Key Points and Best Practices

  1. Scope and Lifetime: Custom properties are scoped to the elements they are defined in and their descendants. This means they can be redefined within child elements, effectively overriding the parent's value.

  2. Inheritance: Custom properties inherit in the same manner as other CSS properties, which means that if a custom property is not defined on an element, it will propagate down the cascade until it finds a valid definition.

  3. Fallbacks: Always provide a fallback value in the var() function to avoid unexpected styling issues if a variable is undefined. This ensures that your styles remain consistent across different parts of your application.

  4. Destructuring and Managing Values: When managing multiple related values, consider creating structured variables that can be easily updated and maintained.

    Example:

    :root {
  --color-theme: #3498db;
  --color-hover: #2980b9;
}

    You can later use --color-theme and --color-hover to keep your color scheme organized and reusable.

  5. Usage with JavaScript: Custom properties can be manipulated using JavaScript, which provides an additional layer of dynamic styling capabilities. This is particularly useful for theming and user-controlled settings.

    Example:

    document.documentElement.style.setProperty('--primary-color', '#ff4d4d');

Conclusion

CSS Custom Properties offer a powerful toolset for enhancing developer productivity and maintaining scalable stylesheets. By utilizing custom properties effectively, developers can minimize repetition, maintain a consistent design system, and ensure that their stylesheets remain organized and easy to manage. As modern web development continues to evolve, CSS Custom Properties will undoubtedly remain a cornerstone of CSS best practices.




CSS Declaring and Using CSS Custom Properties: A Step-By-Step Guide for Beginners

Understanding CSS Custom Properties (often referred to as CSS Variables) can greatly enhance your workflow by allowing you to declare variables that store specific values used throughout your CSS file. Custom properties make it easier to maintain consistency across themes, switch between dark and light themes, or modify design aspects without digging through countless lines of code.

In this guide, we'll walk through declaring, setting a route, running an example application, and observing how data (in our case, style data managed via CSS variables) flows through a simple project. This will give us a comprehensive understanding of CSS Custom Properties from a beginner's perspective.

Step 1: Understanding the Basics

Before diving into practical examples, here are the basics:

  • CSS Custom Properties are defined using -- prefix: For example, a color variable might be defined as --primary-color.
  • They are always local to the element on which they are declared but can inherit down through the DOM tree if not declared as :root.
  • They must be accessed using the var() function in your CSS. For example, color: var(--primary-color);.

Step 2: Setting Up Your Application

For this example, we'll create a simple HTML file with embedded CSS where we will define and use CSS custom properties to manage theme colors and fonts.

Creating the HTML Structure

Create a new file named index.html.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>CSS Custom Properties Example</title>
    <style>
        /* Step 3: Declaring Custom Properties */
        :root {
            --primary-color: #4CAF50;
            --secondary-color: #f44336;
            --font-stack: 'Arial', sans-serif;
        }

        body {
            font-family: var(--font-stack);
            margin: 0;
            padding: 0;
            display: flex;
            justify-content: center;
            align-items: center;
            height: 100vh;
            background-color: var(--secondary-color);
        }

        .card {
            background-color: var(--primary-color);
            padding: 20px;
            border-radius: 8px;
            box-shadow: 0 4px 8px rgba(0, 0, 0, 0.1);
            text-align: center;
        }

        .card h1 {
            color: white;
        }

        .change-theme {
            display: block;
            margin-top: 10px;
            cursor: pointer;
            background-color: transparent;
            border: none;
            color: var(--secondary-color);
            text-decoration: underline;
            font-size: 1em;
        }
    </style>
</head>
<body>

    <div class="card">
        <h1>Welcome to My App</h1>
        <button class="change-theme">Change Theme</button>
    </div>

    <script>
        // Step 4: Implementing Dynamic Theme Change
        document.querySelector('.change-theme').addEventListener('click', () => {
            const root = document.documentElement;
            // Toggle Primary and Secondary Colors
            const primaryColor = getComputedStyle(root).getPropertyValue('--primary-color').trim();
            const secondaryColor = getComputedStyle(root).getPropertyValue('--secondary-color').trim();

            root.style.setProperty('--primary-color', secondaryColor);
            root.style.setProperty('--secondary-color', primaryColor);
        });
    </script>
</body>
</html>

Step 3: Declaring Custom Properties

As you can see in the <style> section, we declare our custom properties within the :root selector, making them globally available:

:root {
    --primary-color: #4CAF50;
    --secondary-color: #f44336;
    --font-stack: 'Arial', sans-serif;
}
  • --primary-color: Sets the main accent color.
  • --secondary-color: Sets the secondary color, often used for buttons or accents.
  • --font-stack: Defines the typeface for our elements.

Each of these variables can now be referenced across the stylesheet using the var() function:

background-color: var(--primary-color);
color: var(--secondary-color);
font-family: var(--font-stack);

Step 4: Running the Application

To run the application, simply open the index.html file in a web browser. You should see a simple card displaying "Welcome to My App" with the specified styles.

Step 5: Changing Themes Dynamically

We've added a button labeled "Change Theme". Clicking this button swaps the --primary-color and --secondary-color values, thus switching between two different themes dynamically.

Here's the JavaScript code handling the theme change:

document.querySelector('.change-theme').addEventListener('click', () => {
    const root = document.documentElement; // Refers to the :root element

    // Fetch current values of the custom properties
    const primaryColor = getComputedStyle(root).getPropertyValue('--primary-color').trim();
    const secondaryColor = getComputedStyle(root).getPropertyValue('--secondary-color').trim();

    // Swap the values by re-setting the CSS variables
    root.style.setProperty('--primary-color', secondaryColor);
    root.style.setProperty('--secondary-color', primaryColor);
});

Explanation Flow:

  1. Event Listener Setup: We start by attaching a click event listener to the .change-theme button.
  2. Obtain Current Styles: Using getComputedStyle(root), we retrieve the current computed styles for the :root element, enabling us to fetch the current values of --primary-color and --secondary-color.
  3. Swap Values: By calling root.style.setProperty method, we redefine the CSS variables (--primary-color and --secondary-color) with swapped values.
  4. Re-rendering: Automatically, all CSS rules referencing these variables re-render, updating the appearance of the page according to the newly assigned values.

Step 6: Observing the Data Flow

  • Initial Load:

    • The page loads with the default CSS variables as stated in the :root.
    • The card appears in #4CAF50 background with text in #f44336 color.

  • Theme Change:

    • After clicking the "Change Theme" button:
      • JavaScript event listener fetches current styles.
      • New CSS variable values are set.
      • Re-rendering occurs, changing background and text colors as per the swapped values.

This example demonstrates how to leverage CSS Custom Properties for more flexible and dynamic styling. By mastering CSS variables, you'll streamline development processes, improve maintainability, and add interactive features to your websites efficiently.

Conclusion

With this step-by-step guide, you've gained a foundational understanding of CSS Custom Properties and practiced their implementation through a practical example. From setting up your project to declaring and using CSS variables, to dynamically manipulating styles via JavaScript, this guide provides you with the tools necessary to incorporate CSS Custom Properties into your web development projects effectively.

Feel free to expand upon this example by adding more complex themes, using media queries to adjust variables for different devices, or implementing more sophisticated interactive features. Happy coding!




Top 10 Questions and Answers on CSS Declaring and Using CSS Custom Properties

1. What are CSS Custom Properties, and how are they different from traditional CSS properties?

Answer: CSS Custom Properties, often referred to as CSS Variables or Custom Variables, allow developers to store values that can be reused throughout a stylesheet. They are declared using the --property naming convention and set with the var() function. Unlike traditional CSS properties, which are predefined by the CSS specification (e.g., background-color, font-size), custom properties are user-defined and versatile.

Example:

:root {
    --primary-color: #3498db;
}

body {
    background-color: var(--primary-color);
}

2. How do you declare a CSS Custom Property?

Answer: Custom properties are declared within a selector, typically using :root for global scope. The :root pseudo-class matches the root element of the document (usually <html>), making the variables available globally throughout the stylesheet.

Example:

:root {
    --header-height: 60px;
    --font-family: 'Arial, sans-serif';
}

Within individual selectors, you can also declare custom properties, making them scoped to that particular selector.

Example:

.button {
    --button-padding: 15px 20px;
    padding: var(--button-padding);
}

3. Can you override a CSS Custom Property value within a different selector?

Answer: Yes, CSS Custom Properties can be overridden within different selectors, similar to how traditional CSS properties work. Overriding the variable within a more specific selector will take precedence over a more general one.

Example:

:root {
    --color: black;
}

.dark-mode {
    --color: white;
}

p {
    color: var(--color); /* Will be black */
}

p.dark-mode-text {
    color: var(--color); /* Will be white due to .dark-mode specificity */
}

4. When is it best to use CSS Custom Properties?

Answer: CSS Custom Properties are beneficial when you need to reuse values across your stylesheet. They are particularly useful for:

  • Consistent theme management (e.g., color schemes).
  • Simplifying the process of maintaining complex designs.
  • Adapting styles dynamically based on user input or breakpoints.

Example:

/* Theme Colors */
:root {
    --main-bg-color: #f0f8ff;
    --main-text-color: #2c3e50;
}

.theme-dark {
    --main-bg-color: #333;
    --main-text-color: #fff;
}

body {
    background-color: var(--main-bg-color);
    color: var(--main-text-color);
}

5. How do you provide a fallback value for a CSS Custom Property?

Answer: You can provide a fallback value for a custom property in the var() function using the second parameter. If the custom property is not defined or invalid, the fallback value will be used.

Example:

:root {
    --primary-font: "Helvetica Neue";
}

body {
    font-family: var(--primary-font, Arial, sans-serif); 
    /* Falls back to Arial, then sans-serif if --primary-font is undefined */
}

6. Is there a way to change the value of a CSS Custom Property dynamically using JavaScript?

Answer: Yes, you can dynamically change CSS Custom Property values via JavaScript by accessing the .style property of the relevant DOM element.

Example:

document.documentElement.style.setProperty('--main-bg-color', '#ffcccb');

This changes the --main-bg-color to #ffcccb globally, affecting all elements that use this property.

7. Can CSS Custom Properties be used in media queries?

Answer: While you cannot directly use CSS Custom Properties inside media queries (they are evaluated at parse-time, not runtime), you can utilize them in conjunction with other CSS features like calc() to create responsive designs.

However, you can define custom properties within media query blocks to change values based on conditions.

Example:

:root {
    --main-padding: 10px;
}

@media (min-width: 600px) {
    :root {
        --main-padding: 20px;
    }
}

section {
    padding: var(--main-padding);
}

8. Are CSS Custom Properties supported in older browsers like Internet Explorer?

Answer: CSS Custom Properties are well-supported in modern browsers but not in Internet Explorer (IE), as it does not support them at all. For projects with a significant dependency on older browsers, fallback mechanisms or polyfills might be necessary.

9. What are some common best practices for using CSS Custom Properties?

Answer:

  • Organization: Group related custom properties logically.
  • Naming conventions: Use descriptive and consistent naming (e.g., --theme-button-color).
  • Scope: Declare properties in the most appropriate scope (e.g., :root, specific components).
  • Fallbacks: Always provide reasonable fallbacks for variables.
  • Performance: Be mindful of the rendering performance of dynamic updates.

10. How can I debug issues arising from the use of CSS Custom Properties?

Answer: Debugging CSS Custom Properties can be simplified by:

  • Checking browser console errors.
  • Using browser DevTools to inspect actual computed values.
  • Ensuring proper scope and overriding rules.
  • Verifying fallback values for undefined variables.
  • Testing with modern and legacy browsers to ensure compatibility.

Example (DevTools Inspection:):

You can inspect custom properties and their values directly in the styles panel of most modern browser developer tools, helping to diagnose any inheritance or assignment issues promptly.

Incorporating CSS Custom Properties can significantly streamline your styling process and enhance the flexibility of your design system. Understanding their nuances and effective usage patterns ensures you can leverage them to their full potential.