First lets get all divs that are on the DOM

const divs = document.getElementsByTagName("div");

You will have array of all the divs that are on the DOM. Then add your class to all of it. In order to do that, lets loop it.

divs.forEach(div => div.className += div.className + " changeClass");

Could this be what you are looking for? In html:

<button onclick="changeColor('blue');">blue</button>

In JS

function changeColor(newColor) {
    var elem = document.getElementsByClassName("changeClass");
    elem.style.color = newColor;
}

The HTML color can be any color you would like it to be, just change they name from blue to any color or input a hex code.

We have multiple divs with the same class value We have given a function to the button that we want the event to happen when it is clicked, using the onclick method. Now when we click the button, the function called myFunction will run.

HTML:

<div class="changeClass">Im Example Div</div>
<div class="changeClass">Me Too</div>
<button type="submit" onclick="myFunction()">Click To Change Div BgColors ! 
</button>

We must define myFunction as Javascript and change the background color.

We have defined a function called myFunction.

With the getElementsByClassName selector in our function, we got all the data with the class value changeClass in object format.

To add a background (or any css property) to all of these objects; We put the object in a for loop and now we split our elements. We can now define a background color for our elements with the style.backgroundColor parameter.

JavaScript:

function myFunction(){
    var divs = document.getElementsByClassName('changeClass');

    for(var i=0; i< divs.length; i++){
        divs[i].style.backgroundColor = 'red';
    }
}

For more detailed information, you can refer to the resources: https://www.w3schools.com/jsref/met_document_getelementsbyclassname.asp