How to change border color of checkboxes in Javascript

I am learning how to do form validation on various types of elements and want to do this in only Javascript. I have some checkboxes here and a Javascript function that checks if the checkboxes has at least one option selected on form submission. Basically the checkboxes should show up red if no option is selected. But I get the error:

Uncaught TypeError: Cannot set property 'borderColor' of undefined

function validate() {
  var ok = true;
  var yes = document.getElementById("yes").checked
  var no = document.getElementById("no").checked;

  if (!yes && !no) {
    document.getElementsByClassName(".btn-group").style.borderColor = "red";
  }

  return ok;
}

<div data-toggle="buttons" class="btn-group">
  <label class="btn active">
    <input id = "yes" type="checkbox" name="box" value="yes" />
    </i>Yes
  </label>

  <label class="btn active">
    <input id = "no" type="checkbox" name="box" value="no" />No
  </label>
</div>