Change css of static class with javascript [duplicate]

I'm trying to change the css rules on a class called active which have static css already but i would like to alter a rule in it to make an div animate slide. i don´t know how to target a class instead of an element with the name of the class.

Javascript

var active = document.getElementsByClassName('active');
active.style.color = 'red';

I´ve tried both these methods but none seems to work.

var i = 0;
    while(i < divs.length) {
        if(divs[i].className == "active") {
            var b = 0;
            for (var a = 0; a < 10; a++){
                divs[i].style.left = b+'em';
                b += 6;

            }
        }
        i++;
    }

I´t wont add left to the css rule nor color.

CSS

.active{
position:absolute;
left:-60em;

Jquery is not an option in this case.