@mixin button ($color) {
  @include sassy-button-structure(rhythm(0.375), rhythm(0.75), rhythm(0.25) rhythm(.75));
  @include sassy-button-gradient('flat', $color, $text-color: gray(30%));
  @include adjust-font-size-to(14px);
  text-shadow: none;
}

@mixin button ($color, $borderWidth: 0, $borderRadius: 3/8, $fontSize: 3/4, $paddingVertical: 1/4, $paddingHorizontal: 3/4) {
  @include sassy-button-structure(rhythm($borderRadius), rhythm($fontSize), rhythm($paddingVertical) rhythm($paddingHorizontal));
  @include sassy-button-gradient('flat', $color, $text-color: gray(30%));
  @include adjust-font-size-to(14px);
  text-shadow: none;

  @if $borderWidth != 0 {
    @include horizontal-borders($borderWidth, $paddingVertical);
    @include apply-side-rhythm-border('left', $borderWidth, $paddingHorizontal);
    @include apply-side-rhythm-border('right', $borderWidth, $paddingHorizontal);
  }
}

Add border option to button mixin
Add border option to button mixin
