<section class="demo-section"> 
<form action="" method="POST" id="demoForm">

<div class="container demo-container">
<div class="demo-image"></div>

<div class="row justify-content-between">

<div class="col-md-3 col-checkbox">
<input type="checkbox" name="demo-checkboxes" id="checkbox-01">
<label for="checkbox-01">Sunglasses</label>
</div>

<div class="col-md-3 col-checkbox">
<input type="checkbox" name="demo-checkboxes" id="checkbox-02">
<label for="checkbox-02">Pencils</label>
</div>

<div class="col-md-3 col-checkbox">
<input type="checkbox" name="demo-checkboxes" id="checkbox-03">
<label for="checkbox-03">Shoes</label>
</div>

<div class="col-md-3 col-checkbox">
<input type="checkbox" name="demo-checkboxes" id="checkbox-04">
<label for="checkbox-04">Notepad</label>
</div>

</div>
</div>

<div id="submit-message"></div>

<div class="text-center submit-button-container">
<input class="btn btn-primary submit-button" type="submit" name="submit" id="submit" value="Submit">
</div>

</form> 
</section>

.demo-container {
  padding: 2em;
  background-image: linear-gradient(to top, #ececec 0%, #ffffff 100%);
}

.demo-image {
  width: 100%;
  height: 340px;
  background-image: url("image.jpg");
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
  background-size: cover;
  margin-bottom: 1em;
}

/* checkboxes */

.col-checkbox input {
  display: none;
}

.col-checkbox label {
  display: block;
  position: relative;
  padding: 10px 0 4px 40px;
  border-bottom: 1px dashed rgba(0, 0, 0, 0.30);
  color: rgba(0, 0, 0, 0.30);
  cursor: pointer;
  text-align: right;
}

.col-checkbox label::before {
  content: '';
  display: block;
  position: absolute;
  top: 10px;
  left: 0;
  width: 24px;
  height: 24px;
  border: 1px solid rgba(0, 0, 0, 0.30);
}

.col-checkbox label:hover, input:focus + label {
  border-bottom: 1px dashed rgb(0, 0, 0, 1.00);
  color: rgba(0, 0, 0, 1.00);
}

.col-checkbox label::before:hover {
  border: 1px solid rgba(0, 0, 0, 0.60);
  color: rgba(0, 0, 0, 1.00);
}

.col-checkbox label:hover::before {
  border: 2px solid rgba(0, 0, 0, 0.60);
} 

.col-checkbox input:checked + label {
  border-bottom: 1px dashed rgb(0, 0, 0, 1.00);
  color: rgba(0, 0, 0, 0.90);
}

.col-checkbox input:checked + label::before {
  position: absolute; 
  font-family: 'FontAwesome';
  font-size: 16px;
  content: '\f00d'; 
  border: 1px solid rgba(0, 0, 0, 0.60);
  color: rgba(0, 0, 255, 0.90);
  font-weight: bold; 
  text-align: center !important;
}

.input-error {
  position: relative;
  text-align: center;
  padding: 4px 20px 0 20px;
  color: rgba(154, 0, 0, 0.60);
}

.input-error::after {
  position: absolute;
  font-family: 'FontAwesome';
  font-size: 20px;
  content: "\f25a";
  top: 5px;
  right: 0;
}

.input-error::before {
  position: absolute;
  font-family: 'FontAwesome';
  font-size: 20px;
  content: "\f25a";
  top: 5px;
  left: 0;
  -moz-transform: scale(-1, 1);
  -webkit-transform: scale(-1, 1);
  -o-transform: scale(-1, 1);
  -ms-transform: scale(-1, 1);
  transform: scale(-1, 1);
}

.submit-button-container {
  margin-top: 1em;
}

.submit-button {
  width: 100%;
}

#submit-message {
  margin-top: 10px;
}

.input-submitted {
  text-align: center;
  color: rgba(0, 102, 49, 0.60)
}

$(function() {
$('#submit').click(function() {

if ($('input[name="demo-checkboxes"]:checked').length == 0) {
$("#submit-message").html('<div class="input-error">Please check at least one item</div>');
$("#submit").addClass("btn-warning");
return false;
} 

else {
$("#submit").removeClass("btn-warning");
return true;
}

});
});