CSS custom properties, also known as CSS variables. represent custom properties that can be declared and be called in your CSS.

Declare a custom property in CSS

To declare a Custom property in your CSS, you need to use the -- syntax:

:root {
  --colorPrimary: hsla(360, 100%, 74%, 0.6);

Notice the :root pseudo-class selector, we can declare our variables globally using it. We can as well declare them using other selectors, they will be then be scoped in these ones.

.theme-dark {
  --colorPrimary: hsla(360, 100%, 24%, 0.6);

Use a custom property in CSS

To use a CSS custom property in your CSS you can use the var() function

:root {
  --colorPrimary: tomato;

.theme-dark {
  --colorPrimary: lime;

body {
  background-color: var(--colorPrimary);

In this case, body will have a background colour of tomato. But body.theme-dark of lime.

Use custom properties without units

CSS custom properties can be declared without units if they are used with the calc() function.

:root {
  --spacing: 2;

.container {
  padding: var(--spacing) px; /*Doesn't Work 😫*/
  padding: calc(var(--spacing) * 1rem); /*Will output 2rem 😃*/

Use custom properties with JavaScript

To get a custom property we can use the following:


// Or if inline

To update the custom property value:

element.style.setProperty("--my-var", newVal);

Example of getting and replacing values:

In the following example, we use the dat.gui controller library to change the value of --scenePerspective, --cubeRotateY, --cubeRotateX custom properties. This method makes it easier to apply a new style, as do not have to apply inline style on each Dom elements.

See the Pen CSS 3D perspective by Vincent Humeau (@vinceumo) on CodePen.