Edit in JSFiddle

<div class="checkbox">
    <input type="checkbox" id="some_checkbox">
    <label for="some_checkbox">Awesome checkbox</label>
</div>

.checkbox input[type=checkbox] {
  position: absolute;
  top: -99999px;
  opacity: 0;
}
.checkbox input[type=checkbox] + label {
  padding-left: 20px;
  position: relative;
}
.checkbox input[type=checkbox] + label::before {
  content: '';
  display: block;
  position: absolute;
  width: 12px;
  height: 12px;
  left: 0;
  top: 0;
  border: 1px solid #999;
  background-color: #ddd;
  border-radius: 3px;
  box-shadow: inset 1px 1px 2px 0 rgba(0, 0, 0, 0.3);
}
.checkbox input[type=checkbox]:checked + label::after {
  content: '';
  display: block;
  position: absolute;
  width: 8px;
  height: 5px;
  left: 2px;
  top: 2px;
  border-bottom: 2px solid #000;
  border-left: 2px solid #000;
  -webkit-transform: rotate(-45deg);
  -ms-transform: rotate(-45deg);
  -o-transform: rotate(-45deg);
  transform: rotate(-45deg);
}