Let's say you have a checkbox with the class (bootstrap) .form-check-input. Then you can use an image for an example as the check mark.
SCSS code:
<input class="form-check-input" type="checkbox">
.form-check-input {
width: 22px;
height: 22px;
-webkit-appearance: none;
-moz-appearance: none;
-o-appearance: none;
appearance:none;
outline: 1px solid blue;
&:checked
{
background: white url('blue.svg') no-repeat;
background-size: 20px 20px;
background-position: 50% 50%;
}
}