mdb-ui-kit/scss/mixins/_forms.scss

@mixin mdb-label-color-toggle-focus() {
  // override bootstrap focus and keep all the standard color (could be multiple radios in the form group)
  .mdb-form-group.is-focused & {
    color: $mdb-label-color;

    // on focus just darken the specific labels, do not turn them to the brand-primary
    &:hover,
    &:focus {
      color: $mdb-label-color-toggle-focus;
    }

    // correct the above focus color for disabled items
    fieldset[disabled] & {
      color: $mdb-label-color;
    }
  }
}

//// Placeholder text
//@mixin material-placeholder() {
//  &::placeholder {
//    @content;
//  }
//  // Firefox
//  //&::-moz-placeholder {@content; } // Firefox
//  //&:-ms-input-placeholder {@content; } // Internet Explorer 10+
//  //&::-webkit-input-placeholder  {@content; } // Safari and Chrome
//}