Bootstrap/scss/mixins/_forms.scss

// Form validation states
//
// Used in _forms.scss to generate the form validation CSS for warnings, errors,
// and successes.

@mixin form-control-validation($color) {
  // Color the label and help text
  .form-control-feedback,
  .form-control-label,
  .col-form-label,
  .form-check-label,
  .custom-control {
    color: $color;
  }

  // Set the border and box shadow on specific inputs to match
  .form-control,
  .custom-select,
  .custom-file-control {
    border-color: $color;

    &:focus {
      @include box-shadow($input-box-shadow, 0 0 0 3px rgba($color, .5));
    }
  }

  // Set validation states also for addons
  .input-group-addon {
    color: $color;
    background-color: lighten($color, 40%);
    border-color: $color;
  }
}

// Form control focus state
//
// Generate a customized focus state and for any input with the specified color,
// which defaults to the `@input-border-focus` variable.
//
// We highly encourage you to not customize the default value, but instead use
// this to tweak colors on an as-needed basis. This aesthetic change is based on
// WebKit's default styles, but applicable to a wider range of browsers. Its
// usability and accessibility should be taken into account with any change.
//
// Example usage: change the default blue border and shadow to white for better
// contrast against a dark gray background.
@mixin form-control-focus() {
  &:focus {
    color: $input-color-focus;
    background-color: $input-bg-focus;
    border-color: $input-border-focus;
    outline: none;
    @include box-shadow($input-box-shadow-focus);
  }
}