Вот простой способ (использовать до или после псевдоэлементов / классов):
input[type=checkbox] {
position: relative;
}
input[type=checkbox]:after {
position: absolute;
top: 0;
left: 0;
/* Above three lines allow the checkbox:after position at checkbox's position */
content: '';
width: 32px;
height: 32px;
z-index: 1; /* This allows the after overlap the checkbox */
/* Anything you want */
}