You can overwrite the --mdb-box-shadow-color-rgb variable, but it only applies to the unchecked checkbox. If you would like to change the other styles, you simply need to overwrite them. This is what the default styles look like:

.form-check-input:checked:focus:before {
  box-shadow: rgb(59, 113, 202) 0px 0px 0px 13px;
  transform: scale(1);
  transition: box-shadow 0.2s ease 0s, transform 0.2s ease 0s;
}

.form-check-input:focus::before {
  opacity: 0.12;
  box-shadow: 0px 0px 0px 13px rgba(var(--mdb-box-shadow-color-rgb, 0.6));
  transform: scale(1);
  transition: box-shadow 0.2s ease 0s, transform 0.2s ease 0s;
}