Bootstrap 4: customize checkbox border

Bootstrap 4 is using a box-shadow to get this effect:

.custom-control-input:focus~.custom-control-indicator {
    -webkit-box-shadow: 0 0 0 1px #fff, 0 0 0 3px #0275d8;
    box-shadow: 0 0 0 1px #fff, 0 0 0 3px #0275d8;
}

The rounded border is defined with border-radius:

.custom-checkbox .custom-control-indicator {
    border-radius: .25rem;
}

To remove the blue border and reset active state background to #fff in Bootstrap v4.0.0 use:

.custom-control-input:focus ~ .custom-control-label::before {
    box-shadow: none;
}
.custom-control-input:active ~ .custom-control-label::before {
    background-color: #fff;
}