_btn.sass

.btn
  line-height: 1
  vertical-align: inherit
  position: relative
  white-space: nowrap
  transition: box-shadow .25s ease-out

  &:after
    content: ''
    opacity: 0
    top: 0
    left: 0
    right: 0
    bottom: 0
    width: 100%
    height: 100%
    display: block
    position: absolute
    background-color: currentcolor
    transition: opacity .25s ease-out

  &:focus
    box-shadow: none

    &:after
      opacity: 0.2

  &:active:after,
  &:hover:after
    opacity: .1
    transition: opacity .25s ease-out

.button, button
  @extend .btn
  @extend .btn-primary