HTML

Checkbox checked border square CSS

<style>
.checkbox-round {
    width: 1.3em;
    height: 1.3em;
    background-color: white;
    border-radius: 50%;
    vertical-align: middle;
    border: 1px solid #ddd;
    appearance: none;
    -webkit-appearance: none;
    outline: none;
    cursor: pointer;
}

.checkbox-round:checked {
    background-color: gray;
}
</style>
<input type="checkbox" class="checkbox-round" />

<demo>
  https://jsfiddle.net/287a4boL
</demo>