SitePoint : Form theming with Sass : From http://www.sitepoint.com/theming-form-elements-sass/

placeholder_theming.sass

$pseudo-phprefix: "::-webkit-input-placeholder" "::-moz-placeholder" ":-ms-input-placeholder" "::placeholder";

$ph-styles: (
  font-family: sans-serif,
  font-size: medium,
  font-style: normal,
  font-weight: normal,
  color: inherit,
  letter-spacing : normal,
  line-height: normal,
  text-align: inherit,
  text-decoration: inherit,
  padding: 0
);

@mixin placeholder-theme($styles) {
  @each $pseudo in $pseudo-phprefix {

    @at-root #{&}#{$pseudo} {
      @each $key, $value in $styles {
        #{$key}: #{$value};
      }
    }

  }
}

@mixin placeholder {
  @each $pseudo in $pseudo-phprefix {

    @at-root #{&}#{$pseudo} {
      @content
    }

  }
}