Use JS to modify properties declared in CSS files. [duplicate]

You can use code like this:

document.querySelector('.test').style.height = '150px';

.test {
  width : 100px;
  height : 100px;
  background : #0AF;
}

<div class="test"></div>

Of course, you have always the opportunity to make the code as abstract as you desire.

In example you can have a function that can work like that:

// Responsible to set the CSS Height property of the given element
function changeHeight( selector, height ) {
    // Choose the element should get modified
    var $element = document.querySelector( selector );
    // Change the height proprety
    $element.style.height = height;
}

changeHeight( '.test', '150px' );

or you can go even more abstract like that:

// Responsible to modify the given CSS property of the given
// HTML element
function changeCssProperty( selector, property, value ) {
    // Find the given element in the DOM
    var $element = document.querySelector( selector );
    // Set the value to the given property
    $element.style[property] = value;
}

changeCssProperty( '.test', 'width', '200px' );