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Jaimal Dullat
Jaimal Dullat

Posted on • Updated on • Originally published at Medium

Using CSS Variables for Dynamic Theming

When it comes to web design, flexibility is key. One of the most significant advancements in web design flexibility in recent years is the introduction of CSS Custom Properties, commonly referred to as CSS variables. With these, designers and developers can introduce dynamic theming and make runtime changes with ease. Let’s dive deep into the magic of CSS variables.

What are CSS Variables?

CSS variables are essentially named entities defined by developers to hold specific values. These can then be reused throughout the CSS document, making it easier to manage and maintain the styles.

For instance, instead of hardcoding a specific color multiple times throughout your CSS, you could set it once in a variable and reference that variable wherever needed.

The Basics: Defining and Using CSS Variables

To define a CSS variable, the syntax involves a double hyphen (--) followed by the variable name.

In style.css (your style file) add the following line to make the main-color variable:

:root {
  --main-color: #3498db;
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To use this variable elsewhere in your CSS:

button {
  background-color: var(--main-color);
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The above code sets a button’s background color to the value stored in --main-color.

Advantages of Using CSS Variables

  1. Centralized Control: By defining colors, fonts, and other attributes in variables, you have a centralized location to manage these values. If you want to change the main color of your website, it’s a matter of updating one line.

  2. Dynamic Theming: CSS variables can be manipulated using JavaScript. This allows for dynamic theme switching, adjusting styles based on user preferences, or even system-wide dark mode settings.

  3. Reduced Redundancy: Instead of repeating the same values in multiple places, you can define them once and reference the variables as needed.

Dynamic Theming and Runtime Changes

With the ability to alter CSS variables through JavaScript, we have the power to change the look of our websites in real-time.

Consider the following:'--main-color', '#e74c3c');
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By executing this JavaScript, we’re updating the --main-color variable to a new value, effectively changing any style that references this variable.

Practical Use Case: Dark Mode

Many modern websites now offer a “dark mode” option. With CSS variables, implementing such a feature becomes a breeze. You could define two sets of color variables — one for the standard light theme and another for the dark theme. Switching between these themes becomes as simple as toggling the values of the variables using JavaScript.

Best Practices

  1. Use Semantic Names: Instead of naming variables like --color1 or --color2, use meaningful names like --header-bg-color or --primary-text-color.

  2. Scope Appropriately: While defining variables in the :root selector makes them globally available, you can also define variables at more specific levels if they are only relevant to a certain section of your site.

  3. Fallback Values: Sometimes, a browser might not have a defined variable (especially if you’re manipulating them with JS). Ensure you provide fallback values: var(--main-color, #3498db)


CSS variables have revolutionized the way we write and manage stylesheets. They offer unparalleled flexibility, making it easier to maintain large CSS files, introduce dynamic theming, and make runtime adjustments. If you haven’t started using CSS variables yet, now is the perfect time to tap into their magic and see the transformative effect they can have on your web projects.

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