CSS Custom Properties (Variables)

CSS Custom Properties or CSS variables allow us to declare variables in our CSS files and use them inside our scope of choice. In this post I will go over some basic use cases for CSS custom properties and finish with an example that combines CSS custom properties and HTML custom attributes.

Declare

A CSS variable is made out of a property name and value:

:root{
  --primary-color: dodgerblue;
}
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Be sure to use the -- prefix before the property name.

Scope

Declaring CSS custom properties on the pseudo :root class is considered a best practice. But you can also declare and use custom properties inside an element of your choice:

:root {
  --gallery-grid-columns: 3;
  --gallery-grid-gap: 15px;
}

.container {
  --primary-color: dodgerblue;
}
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The var() Function

In order to use a CSS custom property we have to use the CSS var() function:

:root {
  --gallery-grid-columns: 3;
  --gallery-grid-gap: 15px;
}

#gallery-container {
  margin: auto;
  display: grid;
  grid-template-columns: repeat(var(--gallery-grid-columns), 1fr);
  grid-gap: var(--gallery-grid-gap);
  padding: 15px;
}

.container {
  --primary-color: dodgerblue;
}

.container p {
  color: var(--primary-color);
}
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Example

My favorite use for CSS custom properties is using JS to manipulate them. I’ve created a pen with a working example that combines CSS custom properties, HTML custom attributes and js: