.form-check-input {
  width: 1em;
  height: 1em;
  border: 2px solid #343a40;      /* borde más oscuro */
  background-clip: content-box;
  padding: 2px;
}

/* checked y foco */
.form-check-input:checked {
  background-color: #0d6efd;
  border-color: #0d6efd;
}
.form-check-input:focus {
  box-shadow: 0 0 0 .2rem rgba(13,110,253,0.25);
}