bootstrap-theme/scss/_ngx-toastr/toastr.scss

.ngx-toastr {
  &.toast-error {
    background-color: $danger;
  }
  &.toast-warning {
    background-color: $warning;
  }
  &.toast-success {
    background-color: $success;
  }
  &.toast-info {
    background-color: $light;
  }
}