Added a custom switch control to a form but it is only displaying a checkbox

I’ve added a custom-switch control to a form, but on my published page it is only displaying a check-box. I’m guessing that I have not yet upload the correct CSS or library file to my server, which file is required? (and where can I find it?)

<div class="col">
                        <div class="custom-control custom-switch">
                          <input class="custom-control-input" type="checkbox" value="" id="checkbox1" name="checkbox1">
                          <label class="custom-control-label" for="checkbox1">Onboard Complete</label>
                        </div>
                      </div>