Assign a CSS variable with Javascript

This is what makes the web platform great, the interconnections between the inner technologies such as CSS and Javascript.

Nowadays, in CSS you can have variables, or more properly named properties. They are great at defining values that need to be changed at run-time.

Those variable' values can be changed both from CSS selectors and Javscript functions as well.

Here the code. Suppose this CSS declaration:

body{
  --myColor: red;
}

p{
  color: var(--myColor);
}

and here the js to change it at run time based on some arbitrary event:

document.body.style.setProperty('--myColor', 'green')

You can change it in the proper DOM element or in any other elements, exploiting the cascading model of CSS as well.