How to style a checkbox using CSS

Answer 1

You can simply use appearance: none on modern browsers, so that there is no default styling and all your styles are applied properly:

input[type=checkbox] {
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  display: inline-block;
  width: 2em;
  height: 2em;
  border: 1px solid gray;
  outline: none;
  vertical-align: middle;
}

input[type=checkbox]:checked {
  background-color: blue;
}