button to toggle css styling / div visibility?

The getElementsByClassName, as the name suggests returns a collection of Elements, meaning an array, so x.style.display won’t work.
x[0].style.display would work for the first element that it finds.
But since this is WordPress, which by default loads Jquery, you could use:

jQuery('.cart-multi-step-button-1').click(function(){
    jQuery('.cart_totals').css({
      'background': 'red',
      'display': 'none',
    });
  });

You could also toggle a class with jQuery, and add the css yourself:

jQuery('.cart_totals').toggleClass('active');

The css:

.cart_totals.active{
  background: red;
  display: none;
}