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Table of Contents
How to Define and Use CSS Variables
2. Use a Variable with var()
Practical Use Cases
? Theme Management (eg, Dark Mode)
? Reusable Design Tokens
Advanced Tips
Scope of CSS Variables
Home Web Front-end CSS Tutorial What are CSS variables and how to use them?

What are CSS variables and how to use them?

Aug 01, 2025 am 06:27 AM

CSS variables (custom attributes) improve style maintainability by defining and reusing values. The answer is to use --define variables and call them with var(). 1. Define them in:root, such as --primary-color: #007bff; 2. Use var() to reference them in styles such as background-color: var(--primary-color); 3. Can be used for topic switching and design token reuse; 4. Support fallback values such as var(--color, #333); 5. Can be read and modified dynamically through JavaScript, and finally realize flexible and maintainable CSS.

What are CSS variables and how to use them?

CSS variables, also known as custom properties , are a way to store and reuse values (like colors, font sizes, spacing, etc.) throughout your stylesheet. They help make your CSS more maintained, consistent, and easier to update.

What are CSS variables and how to use them?

They're especially useful when you're working on large projects or want to implement themes (like light/dark mode) without repeating values or relying on preprocessors like Sass.


How to Define and Use CSS Variables

CSS variables are defined using the -- prefix, and they're usually set inside a CSS rule block—most commonly on the :root pseudo-class so they're globally available.

What are CSS variables and how to use them?

1. Define a Variable

 :root {
  --primary-color: #007bff;
  --font-size-large: 1.5rem;
  --spacing-unit: 16px;
}
  • :root ensures the variables are accessible globally.
  • You can also define variables inside specific selectors for more scoped usage.

2. Use a Variable with var()

Once defined, use the var() function to access the variable:

 .button {
  background-color: var(--primary-color);
  font-size: var(--font-size-large);
  padding: var(--spacing-unit);
}

This will apply the values you defined earlier.

What are CSS variables and how to use them?

Practical Use Cases

? Theme Management (eg, Dark Mode)

You can redefine variables under different conditions using media queries or classes.

 :root {
  --bg-color: #ffffff;
  --text-color: #333333;
}

@media (prefers-color-scheme: dark) {
  :root {
    --bg-color: #1a1a1a;
    --text-color: #f5f5f5;
  }
}

body {
  background-color: var(--bg-color);
  color: var(--text-color);
  transition: background-color 0.3s, color 0.3s;
}

Or toggle themes via JavaScript:

 document.documentElement.style.setProperty('--bg-color', '#000');

? Reusable Design Tokens

Instead of hardcoding values:

 .card {
  border-radius: 8px;
  padding: 16px;
  margin-bottom: 16px;
}

Use variables:

 :root {
  --border-radius-md: 8px;
  --padding-sm: 16px;
  --margin-bottom: 16px;
}

.card {
  border-radius: var(--border-radius-md);
  padding: var(--padding-sm);
  margin-bottom: var(--margin-bottom);
}

Now you can change spacing or radii across your site in one place.


Advanced Tips

  • Fallback values : You can provide a fallback in case the variable is undefined:

     color: var(--text-color, #333); /* #333 is used if --text-color doesn't exist */
  • Variables in variables : You can refer to other variables:

     :root {
      --primary: #007bff;
      --primary-dark: color-mix(in srgb, var(--primary), black 20%);
    }

    Note: color-mix() and other color functions work in modern browsers.

  • JavaScript interaction : CSS variables can be read and changed dynamically:

     // Get a variable
    getComputedStyle(document.documentElement).getPropertyValue('--primary-color');
    
    // Set a variable
    document.documentElement.style.setProperty('--primary-color', 'red');

Scope of CSS Variables

They follow normal CSS cascading rules. Define them on :root for global access, or on specific elements for local scope:

 .sidebar {
  --bg-color: #f0f0f0;
  background: var(--bg-color);
}

This --bg-color only applies within .sidebar and its children.


Basically, CSS variables give you dynamic, reusable values right in plain CSS—no preprocessor needed. They're well-supported in modern browsers and make styling more flexible and maintained.

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